ONEOK INC /NEW/
S-3, 1999-04-15
NATURAL GAS TRANSMISISON & DISTRIBUTION
Previous: WMF GROUP LTD, S-3/A, 1999-04-15
Next: ONEOK INC /NEW/, 8-K, 1999-04-15



<PAGE>
 
    As filed with the Securities and Exchange Commission on April 15, 1999
 
                                                     Registration No. 333-
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
 
                      SECURITIES AND EXCHANGE COMMISSION
                             Washington, DC 20549
 
                                ---------------
 
                                   FORM S-3
                            REGISTRATION STATEMENT
                                     Under
                          The Securities Act of 1933
 
                                ---------------
 
                                  ONEOK, INC.
            (Exact Name of Registrant as Specified in Its Charter)
 
               Oklahoma                              73-1520922
    (State or Other Jurisdiction of               (I.R.S. Employer
    Incorporation or Organization)             Identification Number)
 
                             100 West Fifth Street
                             Tulsa, Oklahoma 74103
                                (918) 588-7000
   (Address, Including Zip Code, and Telephone Number, Including Area Code,
                 of Registrant's Principal Executive Offices)
 
                                James C. Kneale
             Vice President, Chief Financial Officer and Treasurer
                                  ONEOK, Inc.
                             100 West Fifth Street
                             Tulsa, Oklahoma 74103
                                (918) 588-7000
(Name, Address, Including Zip Code, and Telephone Number, Including Area Code,
                             of Agent For Service)
 
                                  Copies to:
            Donald A. Kihle                       Robert A. Yolles
            Gable & Gotwals                  Jones, Day, Reavis & Pogue
   100 West Fifth Street, Suite 1000            77 West Wacker Drive
         Tulsa, Oklahoma 74103                 Chicago, Illinois 60601
            (918) 585-8141                         (312) 782-3939
 
                                ---------------
 
   Approximate date of commencement of proposed sale to the public: From time
to time after this Registration Statement becomes effective as determined by
market conditions and other factors.
 
   If the only securities being registered on this form are being offered
pursuant to dividend or interest reinvestment plans, please check the
following box. [_]
 
   If any of the securities being registered on this form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or
interest reinvestment plans, check the following box. [X]
 
   If this form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following
box and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. [_]
 
   If this form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_]
 
   If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [_]
 
                        CALCULATION OF REGISTRATION FEE
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
<TABLE>
<CAPTION>
                                              Proposed Maximum
            Title Of Each Class                  Aggregate         Amount Of
       Of Securities To Be Registered        Offering Price (1) Registration Fee
- --------------------------------------------------------------------------------
<S>                                          <C>                <C>
Debt Securities (2)........................     $500,000,000        $139,000
</TABLE>
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
(1) Estimated solely for the purpose of calculating the registration fee
    pursuant to Rule 457(o) under the Securities Act. In no event will the
    aggregate initial offering price of all Debt Securities issued from time
    to time pursuant to this Registration Statement exceed $500,000,000. If
    any Debt Securities are issued at an original issue discount, then the
    offering price will be deemed to be the principal amount that results in
    an aggregate initial offering price of up to $500,000,000, less the dollar
    amount of any Debt Securities previously issued hereunder.
(2) There is also being registered hereunder an indeterminate principal amount
    of Debt Securities as may be issuable upon conversion, redemption,
    exchange or exercise of the Debt Securities registered hereunder,
    including pursuant to any applicable antidilution provisions.
   The Registrant hereby amends this Registration Statement on such date or
dates as may be necessary to delay its effective date until the Registrant
shall file a further amendment which specifically states that this
Registration Statement shall thereafter become effective in accordance with
Section 8(a) of the Securities Act of 1933 or until this Registration
Statement shall become effective on such date as the Commission, acting
pursuant to said Section 8(a), may determine.
 
                                ---------------
 
 
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
<PAGE>
 
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+The information in this prospectus is not complete and may be changed. We may +
+not sell these securities until the registration statement filed with the     +
+Securities and Exchange Commission is effective. This prospectus is not an    +
+offer to sell these securities and it is not soliciting an offer to buy these +
+securities in any state where the offer or sale is not permitted.             +
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
       Subject to completion, Preliminary Prospectus dated April 15, 1999
 
                                  $500,000,000
 
                                  ONEOK, Inc.
 
                                Debt Securities
 
                                  -----------
 
  We may offer from time to time unsecured Debt Securities, which may be senior
notes or debentures or other unsecured evidences of indebtedness in one or more
series. The aggregate initial offering price of the Debt Securities that are
offered will not exceed $500,000,000. We will offer the Debt Securities in an
amount and on terms to be determined by market conditions at the time of the
offering.
 
  We will provide specific terms of the particular Debt Securities being
offered in supplements to this prospectus. You should read this prospectus and
any supplement carefully before you invest. This prospectus may not be used to
sell Debt Securities unless accompanied by a prospectus supplement.
 
                                  -----------
 
  Neither the Securities and Exchange Commission nor any state securities
commission has approved or disapproved of these securities or passed upon the
adequacy or accuracy of this prospectus. Any representation to the contrary is
a criminal offense.
 
                                  -----------
 
                 The date of this Prospectus is         , 1999.
<PAGE>
 
                               TABLE OF CONTENTS
 
<TABLE>
<S>                                                                          <C>
About this Prospectus.......................................................   2
Where You Can Find More Information.........................................   2
Forward-Looking Information.................................................   3
About ONEOK.................................................................   4
Use of Proceeds.............................................................   5
Ratio of Earnings to Fixed Charges..........................................   5
Description of Debt Securities..............................................   5
Plan of Distribution........................................................  14
Legal Matters...............................................................  15
Experts.....................................................................  15
</TABLE>
 
                             ABOUT THIS PROSPECTUS
 
   This prospectus is part of a registration statement that ONEOK ("we" or
"ONEOK") filed with the Securities and Exchange Commission using a "shelf"
registration process. Using this process, we may offer the Debt Securities
described in this prospectus in one or more offerings with a total initial
offering price of up to $500,000,000. This prospectus provides you with a
general description of the Debt Securities we may offer. Each time we offer
Debt Securities, we will provide a prospectus supplement and any pricing
supplement that will contain information about the specific terms of that
particular offering. The prospectus supplement or pricing supplement may also
add, update or change information contained in this prospectus. To obtain
additional information that may be important to you, you should read the
exhibits filed by us with this registration statement or our other filings with
the Securities and Exchange Commission. You also should read this prospectus
and any prospectus supplement or pricing supplement together with the
additional information described under the heading "Where You Can Find More
Information."
 
                      WHERE YOU CAN FIND MORE INFORMATION
 
   We file annual, quarterly and special reports, proxy statements and other
information with the Securities and Exchange Commission. You can read and copy
any materials we file with the Securities and Exchange Commission at its Public
Reference Room at 450 Fifth Street, N.W., Washington, D.C. 20549 and at its
regional offices located at Seven World Trade Center, New York, New York 10048
and at 500 West Madison Street, Chicago, Illinois 60661. You can obtain
information about the operations of the Securities and Exchange Commission
Public Reference Room by calling the Securities and Exchange Commission at 1-
800-SEC-0330. The Securities and Exchange Commission also maintains a Web site
that contains information we file electronically with the Securities and
Exchange Commission, which you can access over the Internet at
http://www.sec.gov. Our Common Stock is listed on the New York Stock Exchange
(NYSE: OKE), and you can obtain information about us at the offices of the New
York Stock Exchange, 20 Broad Street, New York, New York 10005.
 
   This prospectus is part of a registration statement we have filed with the
Securities and Exchange Commission relating to the Debt Securities. As
permitted by Securities and Exchange Commission rules, this prospectus does not
contain all of the information we have included in the registration statement
and the accompanying exhibits we file with the Securities and Exchange
Commission. You may refer to the registration statement and the exhibits for
more information about us and the Debt Securities. The registration statement
and the exhibits are available at the Securities and Exchange Commission's
Public Reference Room or through its Web site.
 
   The Securities and Exchange Commission allows us to "incorporate by
reference" the information we file with it, which means that we can disclose
important information to you by referring you to those documents. The
information we incorporate by reference is an important part of this
prospectus, and later
 
                                       2
<PAGE>
 
information that we file with the Securities and Exchange Commission will
automatically update and supersede some of this information. We incorporate by
reference the documents listed below, and any future filings we make with the
Securities and Exchange Commission under Section 13(a), 13(c), 14 or 15(d) of
the Securities Exchange Act of 1934 until we sell all the Debt Securities. The
documents we incorporated by reference are:
 
  .  our annual report on Form 10-K for the year ended August 31, 1998;
 
  .  our quarterly reports on Form 10-Q for the quarterly periods ended
     November 30, 1998 and February 28, 1999; and
 
  .  our current reports on Form 8-K dated September 24, 1998 (as amended on
     October 2, 1998), October 21, 1998, December 16, 1998, December 16,
     1998, January 25, 1999 (as amended on January 26, 1999), January 25,
     1999, February 8, 1999, February 16, 1999, February 23, 1999, February
     24, 1999, February 24, 1999, February 26, 1999, March 5, 1999, March 19,
     1999 and April 15, 1999.
 
   You may request a copy of these filings (other than an exhibit to the
filings unless we have specifically incorporated that exhibit by reference into
the filing), at no cost, by writing or telephoning us at the following address:
 
                                  ONEOK, Inc.
                             100 West Fifth Street
                             Tulsa, Oklahoma 74103
                       Attention: Chief Financial Officer
                           Telephone: (918) 588-7000
 
   You should rely only on the information incorporated by reference or
provided in this prospectus or any prospectus supplement. We have not
authorized anyone else to provide you with different information. We may only
use this prospectus to sell Debt Securities if it is accompanied by a
prospectus supplement. We are only offering these Debt Securities in states
where the offer is permitted. You should not assume that the information in
this prospectus or any applicable prospectus supplement is accurate as of any
date other than the date on the front of those documents.
 
                          FORWARD-LOOKING INFORMATION
 
   Some of the statements contained and incorporated in this prospectus are
forward-looking statements within the meaning of the Private Securities
Litigation Reform Act of 1995. The forward-looking statements relate to
anticipated financial performance, management's plans and objectives for future
operations, business prospects, outcome of regulatory proceedings, market
conditions, the timing of the consummation of our proposed merger with
Southwest Gas Corporation and other matters. The Private Securities Litigation
Reform Act of 1995 provides a safe harbor for forward-looking statements in
various circumstances. The following discussion is intended to identify
important factors that could cause future outcomes to differ materially from
those set forth in the forward-looking statements.
 
   Forward-looking statements include the items identified in the preceding
paragraph, the information concerning possible or assumed future results of
operations and other statements contained or incorporated in this prospectus
identified by words such as "anticipate," "estimate," "expect," "intend,"
"believe," "projection" or "goal."
 
   You should not place undue reliance on the forward-looking statements. They
are based on known and unknown risks, uncertainties and other factors that may
cause our actual results, performance or achievements to be materially
different from any future results, performance or achievements expressed or
implied by the forward-looking statements. Those factors may affect our
operations, markets, products, services and prices. In addition to any
assumptions and other factors referred to specifically in connection with the
forward-looking statements, factors that could cause our actual results to
differ materially from those contemplated in any forward-looking statement
include, among others, the following:
 
                                       3
<PAGE>
 
  .  industry capacity;
 
  .  changes in technology;
 
  .  regulatory delays or conditions imposed by regulatory bodies in
     approving the proposed merger;
 
  .  the effects of weather and other natural phenomena on sales and prices;
 
  .  increased competition from other energy suppliers as well as alternative
     forms of energy;
 
  .  the capital intensive nature of our business;
 
  .  further deregulation, or "unbundling," of the natural gas business;
 
  .  economic climate and growth in the geographic areas in which we do
     business;
 
  .  the uncertainty of gas and oil reserve estimates;
 
  .  the timing and extent of changes in commodity prices for natural gas,
     electricity, crude oil and natural gas liquids;
 
  .  conditions of capital markets and equity markets, including conditions
     that may affect our ability to finance the proposed merger with
     Southwest Gas;
 
  .  Year 2000 issues;
 
  .  the effects of changes in governmental policies and regulatory actions,
     including income taxes, environmental compliance and authorized rates;
     and
 
  .  the other factors listed in the reports we have filed and may file with
     the Securities and Exchange Commission, which are incorporated by
     reference.
 
   Other factors and assumptions not identified above were also involved in the
making of the forward-looking statements. The failure of those assumptions to
be realized, as well as other factors, may also cause actual results to differ
materially from those projected. We have no obligation and make no undertaking
to update publicly or revise any forward-looking statements.
 
                                  ABOUT ONEOK
 
   ONEOK and its subsidiaries engage in several aspects of the energy business.
We purchase, gather, compress, transport and store natural gas for distribution
to consumers. We transport gas for others and lease pipeline capacity to others
for their use in transporting gas. We drill for and produce gas and oil,
extract and sell natural gas liquids, and engage in the gas marketing business.
In addition, we lease and operate a headquarters office building (leasing space
we do not use to others) and own and operate a related parking facility. As a
regulated natural gas utility, we distribute natural gas to approximately 1.4
million customers in the states of Oklahoma and Kansas.
 
   On December 14, 1998, we announced that we had entered into an Agreement and
Plan of Merger providing for the acquisition by merger of Southwest Gas for
cash of approximately $864 million. We will be the survivor of the proposed
merger transaction. The proposed merger is subject to customary conditions,
including approval by the shareholders of Southwest Gas and the receipt of
governmental and other authorizations, and is expected to close later in 1999.
We believe that the requisite regulatory approvals will be received, but we are
uncertain when we will receive those approvals. It is possible that regulatory
approvals that are obtained will contain conditions or limitations that will
adversely affect the results of operations of the combined company. Unaudited
pro forma combined condensed financial data giving effect to the proposed
merger are included in our current report on Form 8-K dated April 15, 1999,
which is incorporated herein by reference. We expect to issue about $660
million of debt and $250 million of trust preferred securities in order to
finance the proposed merger.
 
                                       4
<PAGE>
 
                                USE OF PROCEEDS
 
   Unless we inform you otherwise in the prospectus supplement, we will use the
net proceeds from the sale of the Debt Securities offered by this prospectus
for general corporate purposes. These purposes may include repayment and
refinancing of debt, acquisitions (including our acquisition of Southwest Gas),
working capital, capital expenditures and repurchases and redemptions of
securities. Pending any specific application, we may initially invest funds in
short-term marketable securities or apply them to the reduction of short-term
indebtedness.
 
                       RATIO OF EARNINGS TO FIXED CHARGES
 
   Our ratio of earnings to fixed charges for each of the periods shown is as
follows:
 
<TABLE>
<CAPTION>
                                    For the Years Ended August 31,
      For the Six Months Ended     ----------------------------------------------
         February 28, 1999         1998      1997      1996      1995      1994
      ------------------------     ----      ----      ----      ----      ----
      <S>                          <C>       <C>       <C>       <C>       <C>
               6.58x               5.50x     3.51x     3.28x     2.70x     2.52x
</TABLE>
 
   We have computed the ratios of earnings to fixed charges by dividing
earnings by fixed charges. For this purpose, "earnings" consist of net income
plus fixed charges and income taxes. "Fixed charges" consist of interest
charges, that portion of annual operating lease expense we have deemed to
represent the interest factor and the amortization of debt issue costs.
 
                         DESCRIPTION OF DEBT SECURITIES
 
   The following description states the general terms and provisions of our
unsecured Debt Securities. In this prospectus, "Debt Securities" means the
debentures, notes, bonds and other evidences of indebtedness that we will issue
under an Indenture that we entered into with Chase Bank of Texas, National
Association, as Trustee, on September 24, 1998. Each prospectus supplement that
we provide when we offer Debt Securities will describe the specific terms of
the Debt Securities offered through that prospectus supplement and any general
terms outlined in this section that will not apply to those Debt Securities.
 
   We have summarized various terms and provisions of the Indenture in this
section. The summary is not complete. We also have filed the form of the
Indenture as an exhibit to this registration statement. You should read the
form of Indenture for additional information before you buy any Debt
Securities. The Indenture will be qualified under the Trust Indenture Act of
1939. You should refer to the Trust Indenture Act for provisions that apply to
the Debt Securities. This summary also is subject to and qualified by reference
to the description of the particular terms of the Debt Securities described in
the applicable prospectus supplement or supplements and pricing supplement or
supplements. Capitalized terms used but not defined in this summary have the
meanings specified in the Indenture.
 
   The Debt Securities will be our unsecured obligations and will rank equally
with any of our other senior, unsecured debt. Debt Securities issued under the
Indenture will be issued as part of a series that has been established pursuant
to a supplemental indenture or other corporate action designating the specific
terms of the series of Debt Securities. A prospectus supplement will describe
these terms and will include, among other things, the following:
 
  .  the title of the Debt Securities;
 
  .  the total principal amount of the Debt Securities and the percentage of
     their principal amount at which we will issue the Debt Securities;
 
  .  the date or dates on which the principal of the Debt Securities will be
     payable;
 
  .  the interest rate, the method for determining the interest rate (if the
     interest rate is variable), the date from which interest will accrue,
     interest payment dates and record dates for interest payments;
 
                                       5
<PAGE>
 
  .  the place or places where payments on the Debt Securities will be made,
     where holders may surrender their Debt Securities for transfer or
     exchange and where to serve notices or demands;
 
  .  any provisions for optional redemption or early repayment;
 
  .  any provisions that would obligate us to redeem, purchase or repay Debt
     Securities;
 
  .  whether payments on the Debt Securities will be payable by reference to
     any index, formula or other method;
 
  .  any deletions from, changes of or additions to the events of default or
     covenants described in this prospectus;
 
  .  the portion of the principal amount of Debt Securities that will be
     payable if the maturity is accelerated, if other than the entire
     principal amount;
 
  .  any additional means of defeasance of the Debt Securities, any
     additional conditions or limitations to defeasance of the Debt
     Securities or any changes to those conditions or limitations;
 
  .  any provisions granting special rights to holders of the Debt Securities
     upon the occurrence of events identified in the prospectus supplement;
 
  .  if other than the Trustee, the designation of any Paying Agent or
     Security Registrar for the Debt Securities; and the designation of any
     transfer or other agents or depositories for the Debt Securities;
 
  .  whether we will issue the Debt Securities in individual certificates to
     each holder or in the form of temporary or permanent global securities
     that a depository will hold on behalf of holders;
 
  .  the denominations in which we will issue the Debt Securities or in which
     they may be owned, if other than $1,000 or any integral multiple of
     $1,000;
 
  .  whether and in what circumstances any additional amounts may be payable
     on the Debt Securities to foreign holders; and
 
  .  any other terms or conditions that are consistent with the Indenture.
 
   We may sell the Debt Securities at a discount (which may be substantial)
below their stated principal amount. These discounted Debt Securities may bear
no interest or interest at a rate that at the time of issuance is below market
rates. We will describe in the prospectus supplement any material United States
federal income tax consequences and other special considerations.
 
Restrictive Covenants
 
   We have agreed to two principal restrictions on our activities for the
benefit of holders of the Debt Securities. The restrictive covenants summarized
below will apply to a series of Debt Securities (unless waived or amended) as
long as any of those Debt Securities are outstanding, unless the prospectus
supplement for the series states otherwise. We have used in this summary
description capitalized terms that we have defined below under "--Glossary." In
this description of the covenants only, all references to "us" or "we" mean
ONEOK and our principal subsidiaries, unless the context clearly indicates
otherwise. Our principal subsidiaries are those that own or lease a Principal
Property.
 
   Other than the restrictions on Liens and Sale/Leaseback transactions
described below, the Indenture and the Debt Securities do not contain any
covenants or other provisions designed to protect holders of any Debt
Securities in the event we participate in a highly leveraged transaction. The
Indenture and the Debt Securities also do not contain provisions that give
holders of the Debt Securities the right to require us to repurchase their
securities in the event of a decline in our credit ratings resulting from a
takeover, recapitalization or similar restructuring or otherwise.
 
                                       6
<PAGE>
 
   For these purposes, "debt" includes all notes, bonds, debentures or similar
evidences of obligations for borrowed money.
 
 Limitation on Liens
 
   We have agreed that we will issue, assume or guarantee debt for borrowed
money secured by any Lien upon a Principal Property or shares of stock or debt
of any principal subsidiary only if we secure the Debt Securities equally and
ratably with or prior to the debt secured by that Lien. If we secure the Debt
Securities in this manner, we have the option to secure any of our other debt
or obligations equally and ratably with or prior to the debt secured by the
Lien and, accordingly, equally and ratably with the Debt Securities. This
covenant has exceptions that permit:
 
  .  Liens that existed on the date we first issued a series of Debt
     Securities;
 
  .  Liens on any Principal Property or Restricted Securities of any entity
     existing at the time we merge or consolidate with that entity or acquire
     its property or at the time the entity becomes a principal subsidiary;
 
  .  Liens on any Principal Property existing at the time we acquire that
     Principal Property so long as the Lien does not extend to any of our
     other Principal Property;
 
  .  Liens on any Principal Property, and any Lien on the shares of stock of
     any principal subsidiary formed for the purpose of acquiring that
     Principal Property, either:
 
    .  securing all or part of the cost of acquiring, constructing,
       improving, developing or expanding the Principal Property that was
       incurred before, at or within 12 months after the latest of the
       acquisition or completion of the assets or their commencing
       commercial operation; or
 
    .  securing debt to finance the purchase price of the Principal
       Property or the cost of constructing, improving, developing or
       expanding the assets that was incurred before, at or within 12
       months after the latest of the acquisition or completion of the
       assets or their commencing commercial operation;
 
  .  Liens on any Principal Property or Restricted Securities to secure debt
     owed to us;
 
  .  Liens securing industrial development, pollution control or other
     revenue bonds of a government entity;
 
  .  Liens arising in connection with a project financed with, and securing,
     Non-Recourse Indebtedness;
 
  .  Statutory or other Liens arising in the ordinary course of business and
     relating to amounts that are not delinquent or remain payable without
     penalty or that we are contesting in good faith;
 
  .  Liens (other than Liens imposed by ERISA) on our property incurred or
     required in connection with workmen's compensation, unemployment
     insurance and other social security legislation;
 
  .  Liens securing taxes that remain payable without penalty or that we are
     contesting in good faith if we believe we have adequate reserves for the
     taxes in question;
 
  .  Rights that any governmental entity may have to purchase or order the
     sale of any of our property upon payment of reasonable compensation;
 
  .  Rights that any governmental entity may have to terminate any of our
     franchises, licenses or other rights or to regulate our property and
     business;
 
  .  Liens that we do not assume or on which we do not customarily pay
     interest and that exist on real estate or other rights we acquire for
     sub-station, measuring station, regulating station, gas purification
     station, compressor station, transmission line, distribution line or
     right-of-way purposes;
 
                                       7
<PAGE>
 
  .  Easements or reservations in our property for roads, pipelines, gas
     transmission and distribution lines, electric light and power
     transmission and distribution lines, water mains and other similar
     purposes and zoning ordinances, regulations and restrictions that do not
     impair the use of the property in the operation of our business;
 
  .  Any extensions, renewals, substitutions or replacements of the above-
     described Liens or any debt secured by these Liens if both:
 
    .  the amount of debt secured by the new Lien and not otherwise
       permitted does not exceed the principal amount of debt so secured at
       the time of the renewal or refunding; and
 
    .  the new Lien is limited to the property (plus any improvements)
       secured by the original Lien.
 
   In addition, without securing the Debt Securities as described above, we may
issue, assume or guarantee debt that this covenant would otherwise restrict in
a total principal amount that, when added to all of our other outstanding debt
that this covenant would otherwise restrict and the total amount of
Attributable Debt outstanding for Sale/Leaseback Transactions, does not exceed
a "basket" equal to 15% of Consolidated Net Tangible Assets. When calculating
this total principal amount, we exclude from the calculation Attributable Debt
from Sale/Leaseback Transactions in connection with which we have purchased
property or retired debt as described below under "--Limitation on
Sale/Leaseback Transactions."
 
 Limitation on Sale/Leaseback Transactions
 
   We have agreed that we will enter into a Sale/Leaseback Transaction only if
at least one of the following applies:
 
  .  we could incur debt secured by a Lien on the Principal Property that is
     the subject of that Sale/Leaseback Transaction;
 
  .  the Attributable Debt subject to Sale/Leaseback Transaction would be in
     an amount permitted under the "basket" described above under "--
     Limitation on Liens";
 
  .  the proceeds of that Sale/Leaseback Transaction are used for our
     business and operations; or
 
  .  within the period ending 12 months after the closing of the
     Sale/Leaseback Transaction, we apply the net proceeds of the
     Sale/Leaseback Transaction to the voluntary retirement of any Debt
     Securities or Funded Indebtedness (other than Funded Indebtedness that
     we hold or that is subordinate in right of payment to any Debt
     Securities).
 
 Glossary
 
   "Attributable Debt" means, as to any particular lease under which any Person
is at the time liable, at any date as of which the amount thereof is to be
determined, the total net amount of rent required to be paid by that Person
under the lease during the remaining term thereof (excluding amounts required
to be paid on account of maintenance and repairs, services, insurance, taxes,
assessments, water rates and similar charges and contingent rents), discounted
from the respective due dates thereof at the weighted average of the rates of
interest (or yield to maturity, in the case of Debt Securities originally sold
at a discount) borne by the Debt Securities then outstanding under the
Indenture, compounded annually.
 
   "Consolidated Net Tangible Assets" means (1) the total amount of assets
(less applicable reserves and other properly deductible items) which under GAAP
would be included on a consolidated balance sheet of ONEOK and its subsidiaries
after deducting therefrom (A) all current liabilities, provided, however, that
there will not be deducted billings recorded as revenues deferred pending the
outcome of rate proceedings (less applicable income taxes thereon), if and to
the extent the obligation to refund the same has not been finally determined,
(B) appropriate allowance for minority interests in common stocks of
subsidiaries and (C) all goodwill, trade names, trademarks, patents,
unamortized debt discount and expense and other like intangibles,
 
                                       8
<PAGE>
 
which in each case under GAAP would be included on the consolidated balance
sheet, less (2) the amount which would be so included on the consolidated
balance sheet for investments (less applicable reserves) made in subsidiaries.
 
   "Funded Indebtedness" as applied to any Person, means all debt of that
Person maturing after, or renewable or extendible at that Person's option
beyond, 12 months from the date of determination.
 
   "Lien" means any lien, mortgage, pledge, encumbrance, charge or security
interest securing debt; provided, however, that the following types of
transactions will not be considered for purposes of this definition to result
in a Lien: (1) any acquisition by us of any property or assets subject to any
reservation or exception under the terms of which any vendor, lessor or
assignor creates, reserves or excepts or has created, reserved or excepted an
interest in oil, gas or any other mineral in place or the proceeds thereof, (2)
any conveyance or assignment whereby we convey or assign to any Person or
Persons an interest in oil, gas or any other mineral in place or the proceeds
thereof, (3) any Lien upon any property or assets either owned or leased by us
or in which we own an interest that secures for the benefit of the Person or
Persons paying the expenses of developing or conducting operations for the
recovery, storage, transportation or sale of the mineral resources of the
property or assets (or property or assets with which it is unitized) the
payment to that Person or Persons of our proportionate part of the development
or operating expenses or (4) any hedging arrangements entered into in the
ordinary course of business, including any obligation to deliver any mineral,
commodity or asset in connection with the arrangement.
 
   "Non-Recourse Indebtedness" means, at any time, debt incurred after the date
of the Indenture by us in connection with the acquisition of property or assets
by us or the financing of the construction of or improvements on property,
whenever acquired, provided that, under the terms of that debt and pursuant to
applicable law, the recourse at that time and thereafter of the lenders with
respect to the debt is limited to the property or assets so acquired, or the
construction or improvements, including debt as to which a performance or
completion guarantee or similar undertaking was initially applicable to the
debt or the related property or assets if the guarantee or similar undertaking
has been satisfied and is no longer in effect.
 
   "Person" means any individual, corporation, partnership, limited liability
company, joint venture, association, joint-stock company, trust, unincorporated
organization or government or any agency or political subdivision thereof.
 
   "Principal Property" means any property located in the United States, except
any property that in the opinion of the Board of Directors of ONEOK is not of
material importance to the total business conducted by ONEOK and its
consolidated subsidiaries.
 
   "Sale/Leaseback Transaction" means any arrangement with any Person pursuant
to which we lease any Principal Property that has been or is to be sold or
transferred by us to that Person, other than (1) a lease for a term, including
renewals at the option of the lessee, of not more than three years or
classified as an operating lease under generally accepted accounting
principles, (2) leases between ONEOK and one of our principal subsidiaries or
between principal subsidiaries, (3) leases of a Principal Property executed by
the time of, or within 12 months after the latest of, the acquisition, the
completion of construction or improvement, or the commencement of commercial
operation, of the Principal Property and (4) the ground lease for ONEOK Plaza,
100 West Fifth Street, Tulsa, Oklahoma 74103.
 
Consolidation, Merger and Sale of Assets
 
   The Indenture generally permits a consolidation or merger between us and
another entity. It also permits the sale by us of all or substantially all of
our assets. We have agreed, however, that we will consolidate with or merge
into any entity or transfer or dispose of all or substantially all of our
assets to any entity only if:
 
  .  immediately after giving effect to the transaction, no default or event
     of default would have occurred and be continuing or would result from
     the transaction;
 
                                       9
<PAGE>
 
  .  we are the continuing corporation or, if we are not the continuing
     corporation, the resulting entity is organized and existing under the
     laws of any United States jurisdiction and assumes the due and punctual
     payments on the Debt Securities and the performance of our covenants and
     obligations under the Indenture and the Debt Securities; and
 
  .  we provide the Trustee with a certificate and a legal opinion, each
     stating that the Indenture permits the transaction.
 
   If we engage in any of these transactions that result in any Principal
Property or Restricted Securities becoming subject to any Lien and unless we
are otherwise able to create that Lien, the Indenture provides that the Debt
Securities (so long as those Debt Securities are entitled to the protection of
the "Limitation on Liens" covenant) will be secured to at least the same extent
as the debt that would become secured by the Lien as a result of the
transaction.
 
Events of Default
 
   Unless we inform you otherwise in the prospectus supplement, the following
are events of default for a series of Debt Securities:
 
  .  our failure to pay interest on that series of Debt Securities for 30
     days after it becomes due and payable;
 
  .  our failure to pay principal of or any premium on that series of Debt
     Securities when due;
 
  .  our failure to comply with any of our covenants or agreements for that
     series of Debt Securities or in the Indenture (other than an agreement
     or covenant that we have included in the Indenture solely for the
     benefit of less than all series of Debt Securities) for 60 days after
     the Trustee or the holders of at least 25% in principal amount of all
     outstanding Debt Securities affected by that failure provide written
     notice to us;
 
  .  the default under any agreement under which we have at the time
     outstanding debt in excess of $15,000,000 and, if that debt has not
     already matured, it has been accelerated and the acceleration is not
     rescinded within 30 days after we receive notice from the Trustee or the
     holders of at least 25% in principal amount of all outstanding Debt
     Securities of a series so long as, prior to the entry of judgment in
     favor of the Trustee for payment of the Debt Securities of that series,
     we do not cure the default, or the default under the agreement has not
     been waived;
 
  .  various events involving the bankruptcy, insolvency or reorganization of
     ONEOK; or
 
  .  any other event of default provided for that series of Debt Securities.
 
   A default under one series of Debt Securities will not necessarily be a
default under another series. The Trustee may withhold notice to the holders of
a series of Debt Securities of any default or event of default (except in any
payment on that series of Debt Securities) if the Trustee considers it in the
interest of the holders of that series of Debt Securities to do so.
 
   If an event of default for any series of Debt Securities occurs and is
continuing, the Trustee or the holders of at least 25% in principal amount of
the outstanding Debt Securities of the series affected by the default (or, in
some cases, 25% in principal amount of all Debt Securities affected, voting as
one class) may require us to pay on an accelerated basis the principal of and
all accrued and unpaid interest on those Debt Securities. The holders of a
majority in principal amount of the outstanding Debt Securities of the series
affected by the default (or of all Debt Securities affected, voting as one
class) may in some cases rescind this accelerated payment requirement.
 
   If an event of default occurs and is continuing, the Trustee must use the
degree of care and skill of a prudent man in the conduct of his own affairs.
The Trustee will become obligated to exercise any of its powers
 
                                       10
<PAGE>
 
under the Indenture at the request of any of the holders of any Debt Securities
only after those holders have offered the Trustee indemnity reasonably
satisfactory to it.
 
   The holders of a majority in principal amount of Debt Securities of any
series have the rights to waive past defaults under the Indenture that relate
to that series except for a default in the payment on the Debt Securities or a
provision that can only, under the Indenture, be modified or amended if all
holders that are affected consent.
 
   In most cases, holders of a majority in principal amount of the outstanding
Debt Securities of a series (or of all Debt Securities affected, voting as one
class) may direct the time, method and place of:
 
  .  conducting any proceeding for any remedy available to the Trustee; and
 
  .  exercising any trust or power conferred on the Trustee.
 
   The Indenture requires us to file each year with the Trustee a written
statement as to our compliance with the covenants contained in the Indenture.
 
Modification and Waiver
 
   We may amend or supplement the Indenture if the holders of a majority in
principal amount of the outstanding Debt Securities of all series that the
amendment or supplement affects (acting as one class) consent to it. Without
the consent of the holder of each Debt Security affected, however, no
modification may:
 
  .  reduce the principal of the Debt Security or change its stated maturity;
 
  .  reduce the rate of or change the time for payment of interest on the
     Debt Security;
 
  .  reduce any premium payable on the redemption of the Debt Security or
     change the time at which the Debt Security may or must be redeemed;
 
  .  change any obligation to pay additional amounts on the Debt Security;
 
  .  impair the holder's right to institute suit for the enforcement of any
     payment on the Debt Security;
 
  .  impair the holder's right to convert or exchange any Debt Security;
 
  .  reduce the percentage of principal amount of Debt Securities whose
     holders must consent to an amendment to or supplement of the Indenture;
 
  .  reduce the percentage of principal amount of Debt Securities necessary
     to waive compliance with some of the provisions of the Indenture; or
 
  .  modify provisions relating to amendment or waiver, except to increase
     percentages or to provide that other provisions of the Indenture cannot
     be amended or waived without the consent of each holder affected.
 
   We may amend or supplement the Indenture or waive any provision of it
without the consent of any holders of Debt Securities in various circumstances,
including:
 
  .  to provide for the assumption of our obligations under the Indenture and
     the Debt Securities by a successor;
 
  .  to add covenants that would benefit the holders of any Debt Securities
     or to surrender any of our rights or powers;
 
  .  to add events of default with respect to any Debt Securities;
 
  .  to change or eliminate any provisions only when there are no outstanding
     Debt Securities entitled to the benefit of those provisions;
 
  .  to provide any security for or guarantees of any series of Debt
     Securities;
 
                                       11
<PAGE>
 
  .  to provide for the form or terms of any series of Debt Securities;
 
  .  to appoint a successor Trustee or to provide for the administration of
     the trusts under the Indenture by more than one Trustee;
 
  .  to close the Indenture with respect to the authentication and delivery
     of additional series of Debt Securities;
 
  .  to cure any ambiguity, omission, defect or inconsistency that does not
     adversely affect the interests of the holders of outstanding Debt
     Securities of any series;
 
  .  to make any change to the extent necessary to permit or facilitate
     defeasance or discharge of any series of Debt Securities that does not
     adversely affect the interests of the holders of outstanding Debt
     Securities of any series; or
 
  .  to make any change that does not affect the rights of any holder.
 
   The holders of a majority in principal amount of the outstanding Debt
Securities may waive our obligations to comply with various covenants,
including those relating to (1) our obligation to secure the Debt Securities in
the event of mergers, consolidations and sales of assets, (2) corporate
existence and (3) the restrictions on Liens and Sale/Leaseback Transactions.
 
Defeasance
 
   When we use the term defeasance, we mean discharge from some or all of our
obligations under the Indenture. If we deposit with the Trustee funds or
government securities the maturing principal and interest of which is
sufficient to make payments on the Debt Securities of a series on the dates
those payments are due and payable, then, at our option, either of the
following will occur:
 
  .  we will be discharged from our obligations with respect to the Debt
     Securities of that series ("legal defeasance"); or
 
  .  we will no longer have any obligation to comply with the restrictive
     covenants under the Indenture and any other restrictive covenants that
     apply to that series of the Debt Securities, and the related events of
     default will no longer apply to us ("covenant defeasance").
 
   If we defease a series of Debt Securities, the holders of the Debt
Securities of the series affected will not be entitled to the benefits of the
Indenture, except for our obligations to pay additional amounts, if any, to
provide temporary Debt Securities, to register the transfer or exchange of Debt
Securities, to replace stolen, lost or mutilated Debt Securities or to maintain
paying agencies and hold moneys for payment in trust. In the case of covenant
defeasance, our obligation to pay principal, premium and interest on the Debt
Securities will also survive.
 
   Unless we inform you otherwise in the prospectus supplement, we will be
required to deliver to the Trustee an opinion of counsel that the deposit and
related defeasance would not cause the holders of the Debt Securities to
recognize income, gain or loss for federal income tax purposes. If we elect
legal defeasance, that opinion of counsel must be based upon a ruling from the
Internal Revenue Service or a change in law to that effect.
 
Governing Law
 
   New York law will govern the Indenture and the Debt Securities.
 
Trustee
 
   The Indenture contains limitations on the right of the Trustee, if it
becomes one of our creditors, to obtain payment of claims or to realize on
property received for those claims, as security or otherwise. The Trustee is
 
                                       12
<PAGE>
 
permitted to engage in other transactions with us. If, however, it acquires any
conflicting interest, it must eliminate that conflict or resign.
 
Form, Exchange, Registration and Transfer
 
   We will issue the Debt Securities in registered form, without interest
coupons. We will not charge a service charge for any registration of transfer
or exchange of the Debt Securities. We may, however, require the payment of any
tax or other governmental charge payable for that registration.
 
   Holders may exchange Debt Securities of any series for other Debt Securities
of the same series, the same total principal amount and the same terms but in
different authorized denominations in accordance with the Indenture. Holders
may present Debt Securities for registration of transfer at the office of the
security registrar or any transfer agent we designate. The security registrar
or transfer agent will effect the transfer or exchange when it is satisfied
with the documents of title and identity of the person making the request.
 
   We have appointed the Trustee as security registrar for the Debt Securities.
If a prospectus supplement refers to any transfer agents initially designated
by us, we may at any time rescind that designation or approve a change in the
location through which any transfer agent acts. We are required to maintain an
office or agency for transfers and exchanges in each place of payment. We may
at any time designate additional transfer agents for any series of Debt
Securities.
 
   In the case of any redemption, neither the security registrar nor the
transfer agent will be required to register the transfer or exchange of any
Debt Security either:
 
  .  during a period beginning 15 business days prior to the mailing of the
     relevant notice of redemption and ending on the close of business on the
     day of mailing of that notice; or
 
  .  if we have called the Debt Security for redemption in whole or in part,
     except the unredeemed portion of any Debt Security being redeemed in
     part.
 
Payment and Paying Agents
 
   Unless we inform you otherwise in a prospectus supplement, payments on the
Debt Securities will be made in U.S. dollars at the office of the Trustee. At
our option, however, we may make payments by check mailed to the holder's
registered address or, for global Debt Securities, by wire transfer. Unless we
inform you otherwise in a prospectus supplement, we will make interest payments
to the person in whose name the Debt Security is registered at the close of
business on the record date for the interest payment.
 
   Unless we inform you otherwise in a prospectus supplement, we will designate
the Trustee as our paying agent for payments on Debt Securities issued under
the Indenture. We may at any time designate additional paying agents or rescind
the designation of any paying agent or approve a change in the office through
which any paying agent acts.
 
   Subject to the requirements of any applicable abandoned property laws, the
Trustee and paying agent will pay to us upon written request any money they are
holding for payments on the Debt Securities that remain unclaimed for two years
after the date upon which that payment has become due. After payment to us,
holders entitled to the money must look to us for payment. In that case, all
liability of the Trustee or paying agent with respect to that money will cease.
 
Book-Entry Debt Securities
 
   We may issue the Debt Securities of a series in the form of one or more
global Debt Securities that would be deposited with a depositary or its nominee
identified in the prospectus supplement. We may issue global Debt Securities in
either temporary or permanent form. We will describe in the prospectus
supplement the
 
                                       13
<PAGE>
 
terms of any depositary arrangement and the rights and limitations of owners of
beneficial interests in any global Debt Security.
 
                              PLAN OF DISTRIBUTION
 
   We may sell the Debt Securities offered under this prospectus through
underwriters, agents or dealers or directly to purchasers.
 
Underwriters
 
   The applicable prospectus supplement will identify any agents or
underwriters and describe their compensation (including underwriting discount).
The prospectus supplement will also describe other terms of the offering,
including any discounts or concessions allowed or reallowed or paid to dealers
and any securities exchanges on which the offered Debt Securities may be
listed.
 
   The distribution of Debt Securities under this prospectus may occur from
time to time in one or more transactions at a fixed price or prices, which may
be changed, at market prices prevailing at the time of sale, at prices related
to those prevailing market prices or at negotiated prices.
 
Agents and Direct Sales
 
   If the applicable prospectus supplement indicates, we will authorize dealers
or our agents to solicit offers by various institutions to purchase offered
Debt Securities from us pursuant to contracts that provide for payment and
delivery on a future date. We must approve all institutions, but they may
include, among others:
 
  .  commercial and savings banks;
 
  .  insurance companies;
 
  .  pension funds;
 
  .  investment companies; and
 
  .  educational and charitable institutions.
 
   The institutional purchaser's obligations under a contract will be subject
only to the condition that the purchase of the offered Debt Securities at the
time delivery is allowed by any laws that govern the purchaser. The dealers and
our agents will not be responsible for the validity or performance of the
contracts.
 
General Information
 
   Underwriters, dealers and agents participating in a sale of Debt Securities
may be deemed to be underwriters as defined in the Securities Act of 1933, and
any discounts and commissions received by them and any profit realized by them
on resale of the Debt Securities may be deemed to be underwriting discounts and
commissions under the Securities Act. We may have agreements with the agents,
underwriters and dealers to indemnify them against various civil liabilities,
including liabilities under the Securities Act, or to contribute to payments
that the agents, underwriters or dealers may be required to make as a result of
those civil liabilities.
 
   Unless we indicate differently in a prospectus supplement, we will not list
the Debt Securities on any securities exchange. If we sell a security offered
under this prospectus to an underwriter for public offering and sale, the
underwriter may make a market for that security but is not obligated to do so.
Therefore, we cannot give any assurances to you concerning the liquidity of any
security offered under this prospectus.
 
   Agents and underwriters and their affiliates may be customers of, engage in
transactions with, or perform services for us or our subsidiary companies in
the ordinary course of business.
 
                                       14
<PAGE>
 
                                 LEGAL MATTERS
 
   The validity of the Debt Securities will be passed upon for ONEOK by Gable &
Gotwals, Tulsa, Oklahoma. Any underwriters or agents will be advised about
other issues relating to any offering by Jones, Day, Reavis & Pogue, Chicago,
Illinois. Jones, Day, Reavis & Pogue will rely on Gable & Gotwals as to matters
of Oklahoma law. Jones, Day, Reavis & Pogue from time to time acts as counsel
to ONEOK.
 
                                    EXPERTS
 
   The consolidated financial statements of ONEOK and its subsidiaries as of
August 31, 1998 and 1997, and for each of the years in the three-year period
ended August 31, 1998 have been incorporated by reference herein in reliance
upon the report of KPMG LLP, independent certified public accountants,
incorporated by reference herein, and upon the authority of said firm as
experts in accounting and auditing.
 
   The consolidated financial statements of Southwest Gas and its subsidiaries
as of December 31, 1998 and 1997, and for each of the years in the three-year
period ended December 31, 1998, incorporated by reference in ONEOK's current
report on Form 8-K dated April 15, 1999, incorporated by reference herein, have
been audited by Arthur Andersen LLP, independent public accountants, as
indicated in their report with respect thereto, and which are incorporated by
reference herein in reliance upon the authority of said firm as experts in
giving said report.
 
                                       15
<PAGE>
 
                                    PART II.
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
Item 14. Other Expenses of Issuance and Distribution.
 
   The following table sets forth the expenses in connection with the issuance
and distribution of the securities being registered, other than underwriting
discounts and commissions. All of the amounts shown are estimated, except the
Securities and Exchange Commission registration fee.
 
<TABLE>
      <S>                                                              <C>
      Securities and Exchange Commission registration fee............. $139,000
      Legal fees and expenses.........................................   35,000
      Printing and engraving..........................................   50,000
      Fees of accountants.............................................   40,000
      Fees of Trustee.................................................   15,000
      Rating Agencies Fees............................................  150,000
      Blue sky fees and expenses......................................    1,500
      Miscellaneous...................................................    9,500
                                                                       --------
                                                                       $440,000
                                                                       ========
</TABLE>
 
Item 15. Indemnification of Directors and Officers.
 
   The Company, as an Oklahoma corporation, is empowered by section 1031 of the
Oklahoma General Corporation Act, subject to the procedures and limitations
stated therein, to indemnify any person against expenses (including attorneys'
fees), judgments, fines and amounts paid in settlement actually and reasonably
incurred by him in connection with any threatened, pending or completed action,
suit or proceeding in which such person is made or threatened to be made a
party by reason of his being or having been a director, officer, employee or
agent of the Company. The statute provides that indemnification pursuant to its
provisions is not exclusive of other rights of indemnification to which a
person may be entitled under any bylaw, agreement, vote of shareholders, or
disinterested directors, or otherwise. Article VIII of the By-laws of the
Company provides that directors and officers of the Company shall be
indemnified by the Company to the fullest extent permitted by Oklahoma law as
now or hereafter enforced, including the advance of related expenses. If any
determination is required under applicable law as to whether a director or
officer is entitled to indemnification, such determination shall be made by the
Board, by vote of a quorum of disinterested directors, or by independent legal
counsel by written opinion or by shareholders.
 
   The Certificate of Incorporation of the Company provides that a director of
the corporation shall not be personally liable to the corporation or its
shareholders for monetary damages for breach of fiduciary duty as a director,
except for liability for (1) any breach of the director's duty of loyalty to
the corporation or its shareholders, (2) acts or omissions not in good faith or
which would involve intentional misconduct or a knowing violation of law, (3)
payment of unlawful dividends or unlawful stock purchases or redemptions, or
(4) any transaction from which the director derived an improper personal
benefit.
 
   Pursuant to Article VIII of the By-laws of the Company, upon authorization
and determination (1) by the board of directors by a majority of a quorum
consisting of directors who were not parties to the action, suit, or proceeding
involved; (2) if such a quorum is not obtainable, or even if obtainable and a
quorum of disinterested directors so directs, by independent counsel in a
written opinion; or (3) by the stockholders, the Company is obligated to
indemnify any person who incurs liability by reason of the fact that he is or
was a director, officer, employee, or agent of the Company, or is or was
serving at its request as a director, officer, employee, or agent of another
corporation, partnership, joint venture, trust or other enterprise, or as a
member of any committee or similar body, if he acted in good faith and in a
manner he reasonably believed to be in or not opposed to the best interests of
the Company, and with respect to any criminal action or proceeding, had no
reasonable cause
 
                                      II-1
<PAGE>
 
to believe his conduct was unlawful. However, in an action by or in the right
of the Company, no indemnification will be made if such person shall be
adjudged to be liable to the Company, unless such indemnification is allowed by
a court of competent jurisdiction.
 
   Under an insurance policy obtained by the Company, coverage of Company
officers and directors against liability for neglect, errors, omissions, or
breaches of duty in their capacities as such is provided for both the Company,
to the extent that it is obligated to indemnify such officers and directors,
and the officers and directors themselves. Such coverage is provided in the
amount of $75,000,000, with a retained limit by the Company of $250,000. The
insurance company is obligated to pay any loss in excess of the $250,000
retained limit and defense costs from the first dollar, up to the policy limit
of $75,000,000. Among the policy exclusions are those which exclude coverage
for accounting for profits made within the meaning of Section 16(b) of the
Securities Exchange Act of 1934, claims based upon or attributable to directors
and officers gaining any personal profit or advantage to which such individuals
are not legally entitled, and for any claims brought about or attributable to
the dishonesty of an officer or director.
 
   It is recognized that the above-summarized provisions of the Company's
bylaws and the applicable Oklahoma General Corporation Law may be sufficiently
broad to indemnify officers, directors, and controlling persons of the Company
against liabilities arising under said Act.
 
   The Company and Western Resources, Inc. ("WRI") have entered into a
Registration Rights Agreement which provides for indemnification of the
Company's directors, officers, employees and controlling persons, if any, in
any offering or sale of shares of common stock, obtainable upon conversion of
the Series A Convertible Preferred Stock or Series B Convertible Preferred
Stock, against any claims (including amounts paid in settlement), or actions or
proceedings in respect thereof, arising out of or based upon an untrue
statement or alleged untrue statement of a material fact contained in such
registration statement or prospectus contained therein, or any document
incorporated by reference therein, or arising out of or based upon any omission
or alleged omission to state therein a material fact required to be stated or
necessary to make the statements therein not misleading, in each case only to
the extent that such untrue statement or alleged untrue statement or omission
or alleged omission was made in reliance upon and in conformity with written
information furnished to the Company by WRI or an agent or underwriter thereof
expressly for use therein.
 
 
                                      II-2
<PAGE>
 
Item 16. Exhibits.
 
<TABLE>
<CAPTION>
     Exhibit
     Number    Description
     -------   -----------
     <C>       <S>
      1.1      Form of Underwriting Agreement.
      4.1      Form of Indenture between the Company and Chase Bank of Texas,
               National Association as Trustee, relating to the Securities
               (incorporated by reference to Exhibit 4.1 to the Company's
               registration statement on Form S-3 (Commission File No. 333-
               622279)).
      4.2      Form of First Supplemental Indenture (incorporated by reference
               to Exhibit 5(a) to the Company's current report on Form 8-K
               dated September 24, 1998).
      4.3      Form of Second Supplemental Indenture (incorporated by reference
               to Exhibit 5(b) to the Company's current report on Form 8-K
               dated September 24, 1998).
      4.4      Form of Third Supplemental Indenture (incorporated by reference
               to Exhibit 4 to the Company's current report on Form 8-K dated
               February 8, 1999).
      4.5      Form of Fourth Supplemental Indenture.
      4.6      Form of Debt Security (incorporated by reference to Exhibit 4.2
               to the Company's registration statement on Form S-3 (Commission
               File No. 333-62279)).
      5.1      Opinion of Gable & Gotwals regarding the validity of the Debt
               Securities.
     12.1      Statement of Calculation of Ratio of Earnings to Fixed Charges
               (incorporated by reference to Exhibit 12(a) to the Company's
               current report on Form 8-K dated April 15, 1999).
     23.1      Consent of KPMG LLP.
     23.2      Consent of Arthur Andersen LLP.
     23.3      Consent of Gable & Gotwals (included in Exhibit 5.1).
     24.1      Powers of Attorney (included in the signature page of this
               registration statement).
     25.1      Statement of Eligibility of Chase Bank of Texas, National
               Association under the Trust Indenture Act of 1939 on Form T-1
               relating to the Indenture.
</TABLE>
 
Item 17. Undertakings.
 
   (a) The undersigned registrant hereby undertakes:
 
       (1) To file, during any period in which offers or sales are being
    made, a post-effective amendment to this registration statement:
 
         (i) To include any prospectus required by Section 10(a)(3) of the
      Securities Act of 1933;
 
         (ii) To reflect in the prospectus any facts or events arising
      after the effective date of the registration statement (or the most
      recent post-effective amendment thereof) which, individually or in
      the aggregate, represent a fundamental change in the information set
      forth in the registration statement. Notwithstanding the foregoing,
      any increase or decrease in volume of securities offered (if the
      total dollar value of securities offered would not exceed that which
      was registered) and any deviation from the low or high end of the
      estimated maximum offering range may be reflected in the form of
      prospectus filed with the Commission pursuant to Rule 424(b) if, in
      the aggregate, the changes in volume and price represent no more
      than a 20% change in the maximum aggregate offering price set forth
      in the "Calculation of Registration Fee" table in the effective
      registration statement;
 
         (iii) To include any material information with respect to the
      plan of distribution not previously disclosed in the registration
      statement or any material change to such information in the
      registration statement;
 
 
                                     II-3
<PAGE>
 
  provided, however, that paragraphs (a) (1) (i) and (a) (1) (ii) do not
  apply if the registration statement is on Form S-3, Form S-8 or Form F-3,
  and the information required to be included in a post-effective amendment
  by those paragraphs is contained in periodic reports filed with or
  furnished to the Commission by the registrant pursuant to Section 13 or
  15(d) of the Securities Exchange Act of 1934 that are incorporated by
  reference in the registration statement.
 
     (2) That, for the purpose of determining any liability under the
  Securities Act of 1933, each such post-effective amendment shall be deemed
  to be a new registration statement relating to the securities offered
  therein, and the offering of such securities at that time shall be deemed
  to be the initial bona fide offering thereof.
 
     (3) To remove from registration by means of a post-effective amendment
  any of the securities being registered which remain unsold at the
  termination of the offering.
 
   (b) The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to section 13(a) or section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at the time shall be deemed to be the initial bona fide offering thereof.
 
   (c) Insofar as indemnification for liabilities arising under the Securities
Act of 1933 may be permitted to directors, officers and controlling persons of
the registrant pursuant to the foregoing provisions, or otherwise, the
registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
registrant of expenses incurred or incurred or paid by a director, officer or
controlling person of the registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the Act and
will be governed by the final adjudication of such issue.
 
   (d) The undersigned registrant hereby undertakes that:
 
     (1) for purposes of determining any liability under the Securities Act
  of 1933, the information omitted from the form of prospectus filed as part
  of this registration statement in reliance upon Rule 430A and contained in
  a form of prospectus filed by the registrant pursuant to Rule 424 (b) (1)
  or (4) or 497(h) under the Securities Act shall be deemed to be part of
  this registration statement as of the time it was declared effective.
 
     (2) For the purpose of determining any liability under the Securities
  Act of 1933, each post-effective amendment that contains a form of
  prospectus shall be deemed to be a new registration statement relating to
  the Securities offered therein, and the offering of such Securities at that
  time shall be deemed to be the initial bona fide offering thereof.
 
                                      II-4
<PAGE>
 
                                   SIGNATURES
 
   Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Tulsa, State of Oklahoma, on April 15, 1999.
 
                                          ONEOK, INC.
 
                                                   /s/ James C. Kneale
                                          By: _________________________________
                                                       James C. Kneale
                                               Vice President, Chief Financial
                                                    Officer and Treasurer
 
   KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears
immediately below constitutes and appoints James C. Kneale and John A.
Gaberino, Jr. his or her true and lawful attorney-in-fact and agent, with full
power of substitution and resubstitution, for him or her and in his or her
name, place and stead, in any and all capacities, to sign any and all
amendments (including post-effective amendments) to this registration
statement, and registration statements filed pursuant to Rule 462 under the
Securities Act of 1933, and to file the same with all exhibits thereto and
other documents in connection therewith, with the Securities and Exchange
Commission, granting unto said attorney-in-fact and agents, and each of them,
full power and authority to do and perform each and every act and thing
requisite and necessary to be done, as fully to all intents and purposes as he
or she might or could do in person, hereby ratifying and confirming all that
said attorney-in-fact and agents or any of them, or their or his substitute or
substitutes, may lawfully do or cause to be done by virtue hereof.
 
   Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed below by the following persons in the
capacities and on the dates indicated.
 
<TABLE>
<CAPTION>
             Signature                           Title                    Date
             ---------                           -----                    ----
 
<S>                                  <C>                           <C>
      /s/ Larry W. Brummett          Chairman of the Board, Chief    April 15, 1999
____________________________________  Executive Officer and
         Larry W. Brummett            Director (Principal
                                      Executive Officer)
 
         /s/ W. M. Bell              Director                        April 15, 1999
____________________________________
             W. M. Bell
 
       /s/ D. R. Cummings            Director                        April 15, 1999
____________________________________
           D. R. Cummings
 
      /s/ Barry D. Epperson          Vice President, Controller      April 15, 1999
____________________________________  and Chief Accounting
         Barry D. Epperson            Officer (Principal
                                      Accounting Officer)
 
         /s/ W. L. Ford              Director                        April 15, 1999
____________________________________
             W. L. Ford
 
</TABLE>
 
 
                                      II-5
<PAGE>
 
<TABLE>
<CAPTION>
             Signature                           Title                    Date
             ---------                           -----                    ----
<S>                                  <C>                           <C>
        /s/ H. R. Fricke             Director                        April 15, 1999
____________________________________
            H. R. Fricke
 
         /s/ D. L. Kyle              President, Chief Operating      April 15, 1999
____________________________________  Officer and Director
             D. L. Kyle
 
         /s/ D. T. Lake              Director                        April 15, 1999
____________________________________
             D. T. Lake
 
        /s/ B. H. Mackie             Director                        April 15, 1999
____________________________________
            B. H. Mackie
 
       /s/ James C. Kneale           Vice President, Chief           April 15, 1999
____________________________________  Financial Officer and
          James C. Kneale             Treasurer (Principal
                                      Financial Officer)
 
        /s/ G. D. Parker             Director                        April 15, 1999
____________________________________
            G. D. Parker
 
         /s/ S. L. Young             Director                        April 15, 1999
____________________________________
            S. L. Young
</TABLE>
 
                                      II-6
<PAGE>
 
                                 EXHIBIT INDEX
 
<TABLE>
<CAPTION>
Exhibit
Number   Description
- -------  -----------
<S>      <C>
  1.1    Form of Underwriting Agreement.
 
  4.1    Form of Indenture between the Company and Chase Bank of Texas, National
         Association as Trustee, relating to the Securities (incorporated by reference to
         Exhibit 4.1 to the Company's registration statement on Form S-3 (Commission File
         No. 333-62279)).
 
  4.2    Form of First Supplemental Indenture (incorporated by reference to Exhibit 5(a) to
         the Company's current report on Form 8-K dated September 24, 1998).
 
  4.3    Form of Second Supplemental Indenture (incorporated by reference to Exhibit 5(b)
         to the Company's current report on Form 8-K dated September 24, 1998).
 
  4.4    Form of Third Supplemental Indenture (incorporated by reference to Exhibit 4 to
         the Company's current report on Form 8-K dated February 8, 1999).
 
  4.5    Form of Fourth Supplemental Indenture.
 
  4.6    Form of Debt Security (incorporated by reference to Exhibit 4.2 to the Company's
         registration statement on Form S-3 (Commission File No. 333-62279)).
 
  5.1    Opinion of Gable & Gotwals regarding the validity of the Debt Securities.
 
 12.1    Statement of Calculation of Ratio of Earnings to Fixed Charges (incorporated by
         reference to Exhibit 12(a) to the Company's current report on Form 8-K dated April
         15, 1999).
 
 23.1    Consent of KPMG LLP.
 
 23.2    Consent of Arthur Andersen LLP.
 
 23.3    Consent of Gable & Gotwals (included in Exhibit 5.1).
 
 24.1    Powers of Attorney (included in the signature page of this registration
         statement).
 
 25.1    Statement of Eligibility of Chase Bank of Texas, National Association under the
         Trust Indenture Act of 1939 on Form T-1 relating to the Indenture.
</TABLE>

<PAGE>
 
                                                                     Exhibit 1.1

                                  ONEOK, INC.

                                Debt Securities

                            UNDERWRITING AGREEMENT
                            ----------------------

                                                            ______________, 1999

[Name of Underwriter]

Dear Sirs:

     ONEOK, Inc., an Oklahoma corporation (the "Company"), proposes to issue and
sell an aggregate of $____________ principal amount of the Company's Debt 
Securities (the "Securities") to be issued pursuant to an Indenture, dated as of
September 24, 1998, as amended or supplemented (the "Indenture"), between the 
Company and Chase Bank of Texas, National Association, as Trustee (the 
"Trustee"). The Securities will be sold to ______________ (the "Underwriter" or 
"you").

     The purchase price for the Securities to be paid by you shall be agreed 
upon by the Company and the Underwriter, and such agreement shall be set forth 
in a separate written instrument substantially in the form of Exhibit A hereto 
(the "Price Determination Agreement"). The Price Determination Agreement may 
take the form of an exchange of any standard form of written telecommunication 
between the Company and the Underwriter and shall specify such applicable 
information as is indicated in Exhibit A hereto. The offering of the Securities
will be governed by this Agreement, as supplemented by the Price Determination 
Agreement. From and after the date of the execution and delivery of the Price 
Determination Agreement, this Agreement shall be deemed to incorporate, and, 
unless the context otherwise indicates, all references contained herein to "this
Agreement" and to the phrase "herein" shall be deemed to include the Price 
Determination Agreement.

     The Company confirms as follows its agreement with the Underwriter.

     1.  Agreement to Sell and Purchase. On the basis of the representations, 
warranties and agreements of the Company herein contained and subject to all the
terms and conditions of this Agreement, the Company agrees to sell to the 
Underwriter, and the Underwriter agrees to purchase from the Company, the 
principal amount of the Securities, all at the purchase price plus accrued 
interest, if any, from ________, 1999, to the Closing Date (as hereinafter 
defined), to
<PAGE>
 
be agreed upon by the Underwriter and the Company and set forth in the Price
Determination Agreement.

     2.   Delivery and Payment.  Delivery of the Securities shall be made to the
Underwriter against payment of the purchase price by wire transfer in
immediately available funds to an account specified by the Company. Such payment
shall be made at 10:00 a.m., New York City time, on the third business day after
the date on which the first bona fide offering of the Securities to the public
is made by you or at such time on such other date, not later than 10 business
days after such date as may be agreed upon by the Company and you (such date is
hereinafter referred to as the "Closing Date"). Delivery of the Securities shall
be made through the facilities of the Depository Trust Company unless you shall
otherwise instruct.

     3.   Representations and Warranties of the Company. The Company represents,
warrants and covenants to the Underwriter that:

          (a) The Company meets the requirements for use of Form S-3 and a
registration statement (Registration No. 333-______) on Form S-3 relating to the
Securities including a preliminary prospectus and such amendments to such
registration statement as may have been required to the date of this Agreement,
has been prepared by the Company under the provisions of the Securities Act of
1933, as amended (the "Act"), and the rules and regulations (collectively
referred to as the "Rules and Regulations") of the Securities and Exchange
Commission (the "Commission") thereunder, and has been filed with the
Commission. The term "preliminary prospectus" as used herein means a preliminary
prospectus as contemplated by Rule 430 or Rule 430A ("Rule 430A") of the Rules
and Regulations included at any time as part of the registration statement.
Copies of such registration statement and amendments and of each related
preliminary prospectus have been delivered to you. The term "Registration
Statement" means the registration statement as amended at the time it became
effective (the "Effective Date"), including financial statements and all
exhibits and any information deemed to be included by Rule 430A or Rule 434 of
the Rules and Regulations. If the Company files a registration statement to
register a portion of the Securities and relies on Rule 462(b) of the Rules and
Regulations for such registration statement to become effective upon filing with
the Commission (the "Rule 462 Registration Statement"), then any reference to
the "Registration Statement" shall be deemed to include the Rule 462
Registration Statement, as amended from time to time. The term "Prospectus"
means the prospectus as first filed with the Commission pursuant to Rule 424(b)
of the Rules and Regulations in connection with the offering of the Securities
and the form of final prospectus included in the Registration Statement at the
Effective Date. Any reference herein to the Registration Statement, any
preliminary prospectus or the Prospectus shall be deemed to refer to and include
the documents incorporated by reference therein pursuant to Item 12 of Form S-3
which were filed under the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), on or before the Effective Date or the date of such preliminary
prospectus or the Prospectus, as the case may be. Any reference herein to the
terms "amend," "amendment" or "supplement" with respect to the Registration
Statement, any preliminary prospectus or the Prospectus shall be deemed to refer
to and include the filing of any document under the Exchange Act after the
Effective Date, or the date of any preliminary prospectus or the Prospectus, as
the case may be, and deemed to be incorporated therein by reference.

                                       2
<PAGE>
 
          (b) On the Effective Date, the date the Prospectus is first filed with
the Commission pursuant to Rule 424(b) (if required), at all times subsequent to
and including the Closing Date and when any post-effective amendment to the
Registration Statement becomes effective or any amendment or supplement to the
Prospectus is filed with the Commission, the Registration Statement and the
Prospectus (as amended or as supplemented if the Company shall have filed with
the Commission any amendment or supplement thereto), including the financial
statements included or incorporated by reference in the Prospectus, did or will
comply with the applicable provisions of the Act, the Exchange Act, the rules
and regulations thereunder (the "Exchange Act Rules and Regulations"), the Trust
Indenture Act of 1939, as amended (the "Trust Indenture Act"), the rules and
regulations thereunder (the "Trust Indenture Act Rules and Regulations") and the
Rules and Regulations and will contain all statements required to be stated
therein in accordance with the Act, the Exchange Act, the Exchange Act Rules and
Regulations and the Rules and Regulations. On the Effective Date and when any
post-effective amendment to the Registration Statement becomes effective, no
part of the Registration Statement, or any such amendment did or will contain
any untrue statement of a material fact or omit to state a material fact
necessary to make the statements therein not misleading. At the Effective Date,
the date the Prospectus or any amendment or supplement to the Prospectus is
filed with the Commission and at the Closing Date, the Prospectus did not or
will not contain any untrue statement of a material fact or omit to state a
material fact necessary to make the statements therein, in the light of the
circumstances under which they were made, not misleading. The foregoing
representations and warranties in this Section 3(b) do not apply to any
statements or omissions made in reliance on and in conformity with information
relating to the Underwriter furnished in writing to the Company by you
specifically for inclusion in the Registration Statement or Prospectus or any
amendment or supplement thereto. For all purposes of this Agreement, the amounts
of the selling concession and reallowance set forth in the Prospectus under the
caption "Underwriting" constitute the only information relating to the
Underwriter furnished in writing to the Company by you specifically for
inclusion in the Registration Statement or the Prospectus. The Company has not
distributed any offering material in connection with the offering or sale of the
Securities other than the Registration Statement, the preliminary prospectus,
the Prospectus or any other materials, if any, permitted by the Act. On the
Effective Date, the date the Prospectus is first filed with the Commission
pursuant to Rule 497 (if required), and at all subsequent times to and including
the Closing Date, the Indenture did or will comply with all applicable
provisions of the Trust Indenture Act and the Trust Indenture Act Rules and
Regulations.

          (c) The documents which are incorporated by reference in the
Prospectus or from which information is so incorporated by reference, when they
become effective or were filed with the Commission, as the case may be, complied
in all material respects with the requirements of the Act or the Exchange Act,
as applicable, the Exchange Act Rules and Regulations and the Rules and
Regulations; and any documents so filed and incorporated by reference subsequent
to the date hereof shall, when they are filed with the Commission, conform in
all material respects with the requirements of the Act and the Exchange Act, as
applicable, the Exchange Act Rules and Regulations and the Rules and
Regulations.

          (d) The only subsidiaries (as defined in the Rules and Regulations) of
the Company are the subsidiaries listed on Exhibit 21 to the Company's Annual
Report on Form 10-K for the fiscal year ended August 31, 1998 (the
"Subsidiaries"). The Company and each of its Subsidiaries is, and at the Closing
Date will be, a corporation duly organized, validly

                                       3
<PAGE>
 
existing and in good standing under the laws of its jurisdiction of
incorporation. The Company and each of its Subsidiaries has, and at the Closing
Date will have, full power and authority to conduct all the activities conducted
by it, to own or lease all the assets owned or leased by it and to conduct its
business as described in the Registration Statement and the Prospectus. The
Company and each of its Subsidiaries is, and at the Closing Date will be, duly
licensed or qualified to do business and in good standing as a foreign
corporation in all jurisdictions in which the nature of the activities conducted
by it or the character of the assets owned or leased by it makes such licensing
or qualification necessary, except to the extent that the failure to so qualify
or be in good standing would not have a material adverse effect on the condition
(financial or otherwise), earnings, business affairs or business prospects of
the Company and its Subsidiaries, considered as one enterprise (a "Material
Adverse Effect"). All of the outstanding shares of capital stock or capital
interests of the Subsidiaries have been duly authorized and validly issued and
are fully paid and non-assessable and are owned by the Company free and clear of
all liens, encumbrances and claims whatsoever. Except for the stock of the
Subsidiaries and as disclosed in the Registration Statement, the Company does
not own, and at the Closing Date will not own, directly or indirectly, any
shares of stock or any other equity or long-term debt securities of any
corporation or have any equity interest in any firm, partnership, joint venture,
association or other entity. Complete and correct copies of the certificate of
incorporation and of the by-laws of the Company and each of its Subsidiaries and
all amendments thereto have been delivered to you, and no changes therein will
be made subsequent to the date hereof and prior to the Closing Date.

          (e) The Securities have been duly and validly authorized and, when
authenticated by the Trustee and issued, delivered and sold in accordance with
this Agreement and the Indenture, will have been duly and validly executed,
authenticated, issued and delivered and will constitute valid and binding
obligations of the Company, enforceable against the Company in accordance with
their respective terms and entitled to the benefits provided by the Indenture.
At the Closing Date, the Company will have an authorized and outstanding
capitalization as set forth in the Prospectus.

          (f) The description of the Securities in the Registration Statement
and the Prospectus is, and at the Closing Date will be, complete and accurate in
all respects. The Indenture conforms to the description thereof contained in the
Registration Statement and the Prospectus.

          (g) The financial statements and schedules included or incorporated by
reference in the Registration Statement or the Prospectus present fairly the
consolidated financial condition of the Company or Southwest Gas Corporation, a
California corporation ("Southwest Gas"), as the case may be, as of the
respective dates thereof and the consolidated results of operations and cash
flows of the Company or Southwest Gas, as the case may be, for the respective
periods covered thereby, all in conformity with generally accepted accounting
principles applied on a consistent basis throughout the entire period involved,
except as otherwise disclosed in the Prospectus. The pro forma financial
statements and other pro forma financial information included in the
Registration Statement or the Prospectus (i) present fairly in all material
respects the information shown therein, (ii) have been prepared in accordance
with the Commission's rules and guidelines with respect to pro forma financial
statements and (iii) have been properly computed on the bases described therein.
The assumptions used in the preparation of the pro forma financial statements
and other pro forma financial information

                                       4
<PAGE>
 
included in the Registration Statement or the Prospectus are reasonable and the
adjustments used therein are appropriate to give effect to the transactions or
circumstances referred to therein. No other financial statements or schedules of
the Company or Southwest Gas are required by the Act, the Exchange Act or the
Rules and Regulations to be included in the Registration Statement or the
Prospectus. KPMG LLP and Arthur Andersen LLP (the "Accountants"), who have
reported on such financial statements and schedules, are independent accountants
with respect to the Company and Southwest Gas, respectively, as required by the
Act and the Rules and Regulations. There are no statements required to be
included in the Registration Statement or the Prospectus with respect to the
Accountants pursuant to Rule 509 of Regulation S-K of the Rules and Regulations.

          (h) The Company maintains a system of internal accountings control
sufficient to provide reasonable assurance that (i) transactions are executed in
accordance with management's general or specific authorization; (ii)
transactions are recorded as necessary to permit preparation of financial
statements in conformity with generally accepted accounting principles and to
maintain accountability for assets; (iii) access to assets is permitted only in
accordance with management's general or specific authorization; and (iv) the
recorded accountability for assets is compared with existing assets at
reasonable intervals and appropriate action is taken with respect to any
differences.

          (i) Subsequent to the respective dates as of which information is
given in the Registration Statement and the Prospectus and prior to the Closing
Date, except as set forth in or contemplated by the Registration Statement and
the Prospectus, (i) there has not been and will not have been any change in the
capitalization of the Company, or in the business, properties, business
prospects, condition (financial or otherwise) or results of operations of the
Company and its Subsidiaries, arising for any reason whatsoever, (ii) neither
the Company nor any of its Subsidiaries has incurred nor will it incur any
material liabilities or obligations, direct or contingent, nor has it entered
into nor will it enter into any material transactions other than pursuant to
this Agreement and the transactions referred to herein and (iii) the Company has
not and will not have paid or declared any dividends or other distributions of
any kind on any class of its capital stock, other than the ordinary quarterly
dividend paid or payable by the Company to holders of its common stock and
preferred stock consistent with past practices.

          (j) The Company is not an "investment company" or an "affiliated
person" of, or "promoter" or "principal underwriter" for, an "investment
company," as such terms are defined in the Investment Company Act of 1940, as
amended.

          (k) Except as set forth in the Registration Statement and the
Prospectus, there are no actions, suits or proceedings pending or threatened
against or affecting the Company or any of its Subsidiaries or any of their
respective directors or officers in their capacity as such, before or by any
Federal or state court, commission, regulatory body, administrative agency or
other governmental body, domestic or foreign, wherein an unfavorable ruling,
decision or finding might have a Material Adverse Effect.

          (l) The Company and each of its Subsidiaries has, and at the Closing
Date will have, (i) all governmental licenses, permits, consents, orders,
approvals and other authorizations (collectively, "Governmental Licenses")
necessary to carry on its business as contemplated in the Prospectus, except for
the Governmental Licenses the absence of which

                                       5
<PAGE>
 
would not have a Material Adverse Effect, (ii) complied in all material respects
with all laws, regulations and orders applicable to it or its business and (iii)
performed all its material obligations required to be performed by it, and is
not, and at the Closing Date will not be, in default, under any indenture,
mortgage, deed of trust, voting trust agreement, loan agreement, bond,
debenture, note agreement, lease, contract or other agreement or instrument
(collectively, a "contract or other agreement") to which it is a party or by
which its property is bound or affected, other than defaults that would not,
individually or in the aggregate, result in a Material Adverse Effect. To the
best knowledge of the Company and each of its Subsidiaries, no other party under
any contract or other agreement to which it is a party is in material default in
any respect thereunder. Neither the Company nor any of its Subsidiaries is, nor
at the Closing Date will any of them be, in violation of any provision of its
certificate of incorporation or by-laws.

          (m) No consent, approval, authorization or order of, or any filing or
declaration with, any court or governmental agency or body is required in
connection with the authorization, issuance, transfer, sale or delivery of the
Securities by the Company, in connection with the execution, delivery and
performance of this Agreement by the Company or in connection with the taking by
the Company of any action contemplated hereby and in the Indenture and the
Securities, except such as have been obtained under the Act, the Trust Indenture
Act, the Trust Indenture Act Rules and Regulations or the Rules and Regulations
and such as may be required under state securities or Blue Sky laws or the by-
laws and rules of the National Association of Securities Dealers, Inc. (the
"NASD") in connection with the purchase and distribution by the Underwriter of
the Securities.

          (n) The Company has full corporate power and authority to enter into
this Agreement. This Agreement has been duly authorized, executed and delivered
by the Company and constitutes a valid and binding agreement of the Company and
is enforceable against the Company in accordance with the terms hereof. The
Indenture has been duly authorized and constitutes a valid and binding agreement
of the Company and is enforceable against the Company in accordance with its
terms. The performance by the Company of this Agreement, the Indenture and the
Securities and the consummation of the transactions contemplated hereby and
thereby and the application of the net proceeds from the offering and sale of
the Securities to be sold by the Company in the manner set forth in the
Prospectus under "Use of Proceeds" will not result in the creation or imposition
of any lien, charge or encumbrance upon any of the assets of the Company or any
of its Subsidiaries pursuant to the terms or provisions of, or result in a
breach or violation of any of the terms or provisions of, or constitute a
default under, or give any other party a right to terminate any of its
obligations under, or result in the acceleration of any obligation under, the
certificate of incorporation or by-laws of the Company or any of its
Subsidiaries, any material contract or other agreement to which the Company or
any of its Subsidiaries is a party or by which the Company or any of its
Subsidiaries or any of its properties is bound or affected, or violate or
conflict with any judgment, ruling, decree, order, statute, rule or regulation
of any court or governmental agency or body applicable to the business or
properties of the Company or any of its Subsidiaries.

          (o) The Company and each of its Subsidiaries has good and marketable
title to all properties and assets described in the Prospectus as owned by it,
free and clear of all liens, charges, encumbrances or restrictions, except such
as are described in the Prospectus or are not material to the business of the
Company or its Subsidiaries. The Company and each of its Subsidiaries has valid,
subsisting and enforceable leases for the properties described in the

                                       6
<PAGE>
 
Prospectus as leased by it, with such exceptions as are not material and do not
materially interfere with the use made and proposed to be made of such
properties by the Company and such Subsidiaries.

          (p) There is no document or contract of a character required to be
described in the Registration Statement or the Prospectus or to be filed as an
exhibit to the Registration Statement which is not described or filed as
required. All such contracts to which the Company or any Subsidiary is a party
have been duly authorized, executed and delivered by the Company or such
Subsidiary, constitute valid and binding agreements of the Company or such
Subsidiary and are enforceable against the Company or such Subsidiary in
accordance with the terms thereof.

          (q) No statement, representation, warranty or covenant made by the
Company in this Agreement or the Indenture or made in any certificate or
document required by this Agreement to be delivered to you was or will be, when
made, inaccurate, untrue or incorrect in any material respect.

          (r) Neither the Company nor any of its directors, officers or
controlling persons has taken, directly or indirectly, any action designed, or
which might reasonably be expected, to cause or result, under the Act or
otherwise, in, or which has constituted, stabilization or manipulation of the
price of any security of the Company to facilitate the sale or resale of the
Securities.

          (s) No holder of securities of the Company has rights to the
registration of any securities of the Company because of the filing of the
Registration Statement.

          (t) The Company and its Subsidiaries are in substantial compliance
with all federal, state and local employment and labor laws, including, but not
limited to, laws relating to non-discrimination in hiring, promotion and pay of
employees; no labor dispute with the employees of the Company or any Subsidiary
exists or, to the knowledge of the Company, is imminent or threatened; and the
Company is not aware of any existing, imminent or threatened labor disturbance
by the employees of any of its principal suppliers, manufacturers or contractors
that could result in a Material Adverse Effect.

          (u) The Company and its Subsidiaries own, or are licensed or otherwise
have the full exclusive right to use, all material trademarks and trade names
which are used in or necessary for the conduct of their respective businesses as
described in the Prospectus. No claims have been asserted by any person to the
use of any such trademarks or trade names or challenging or questioning the
validity or effectiveness of any such trademark or trade name. The use, in
connection with the business and operations of the Company and its Subsidiaries
of such trademarks and trade names does not, to the Company's knowledge,
infringe on the rights of any person.

          (v) Neither the Company nor any of its Subsidiaries nor, to the
Company's knowledge, any employee or agent of the Company or any Subsidiary has
made any payment of funds of the Company or any Subsidiary or received or
retained any funds in violation of any law, rule or regulation or of a character
required to be disclosed in the Prospectus.

                                       7
<PAGE>
 
          (w) All United States federal income tax returns of the Company and
its Subsidiaries required by law to be filed have been filed, and all other
franchise and income tax returns of the Company and its Subsidiaries required to
be filed pursuant to applicable foreign, state or local law have been filed, and
all taxes shown by such returns or otherwise assessed, which are due and
payable, have been paid, except tax assessments, if any, as are being contested
in good faith and as to which adequate reserves have been provided. The charges,
accruals and reserves on the books of the Company and its Subsidiaries in
respect of any income and corporate franchise tax liability for any years not
finally determined are adequate to meet any assessments or reassessments for
additional income or corporate franchise tax for any years not finally
determined.

          (x) The Company and its subsidiaries are implementing a comprehensive,
detailed program to analyze and address the risk that the computer hardware and
software used by them may be unable to recognize and properly execute date
sensitive functions involving certain dates prior to and any dates after
December 31, 1999 (the "Year 2000 Problem"), and reasonably believes that such
risk will be remedied on a timely basis without material expense and will not
have a Material Adverse Effect; and the Company believes, after due inquiry,
that each supplier, vendor, customer or financial service organization used or
serviced by the Company and its Subsidiaries has remedied or will remedy on a
timely basis the Year 2000 Problem, except to the extent that a failure to
remedy by any such supplier, vendor, customer or financial service organization
would not have a Material Adverse Effect. The Company is in compliance with the
Commission's most recent staff legal bulletin related to Year 2000 compliance.

          (y) The Company and its Subsidiaries (i) are in compliance with any
and all applicable foreign, federal, state and local laws and regulations
relating to the protection of human health and safety, the environment or
imposing liability or standards of conduct concerning any Hazardous Material (as
hereinafter defined) ("Environmental Laws"), (ii) have received all permits,
licenses or other approvals required of them under applicable Environmental Laws
to conduct their respective businesses and (iii) are in compliance with all
terms and conditions of any such permit, license or approval, except where such
noncompliance with Environmental Laws, failure to receive required permits,
licenses or other approvals or failure to comply with the terms and conditions
of such permits, licenses or approvals would not, individually or in the
aggregate, result in a Material Adverse Effect. The term "Hazardous Material"
means (A) any "hazardous substance" as defined by the Comprehensive
Environmental Response, Compensation and Liability Act of 1980, as amended, (B)
any "hazardous waste" as defined by the Resource Conservation and Recovery Act,
as amended, (C) any petroleum or petroleum product, (D) any polychlorinated
biphenyl and (E) any pollutant or contaminant or hazardous, dangerous, or toxic
chemical, material, waste or substance regulated under or within the meaning of
any other Environmental Law.

          (z) In the ordinary course of its business, the Company conducts a
periodic review of the effect of Environmental Laws on the business, operations
and properties of the Company and its Subsidiaries, in the course of which it
identifies and evaluates associated costs and liabilities (including, without
limitation, any capital or operating expenditures required for clean-up, closure
of properties or compliance with Environmental Laws or any permit, license or
approval, any related constraints on operating activities and any potential
liabilities to third parties). Except as set forth in the Registration Statement
and the Prospectus, there are no costs

                                       8
<PAGE>
 
and liabilities associated with or arising in connection with Environmental Laws
as currently in effect (including, without limitation, costs of compliance
therewith) which would, singly or in the aggregate have a Material Adverse
Effect.

          (aa) The Company maintains insurance with respect to its properties
and business of the types and in amounts generally deemed adequate for its
business and consistent with insurance coverage maintained by similar companies
and businesses, all of which insurance is in full force and effect.

          (bb)   With respect to each employee benefit plan, program and
arrangement (including, without limitation, any "employee benefit plan" as
defined in Section 3(3) of the Employee Retirement Income Security Act of 1974,
as amended ("ERISA")) maintained or contributed to by the Company, or with
respect to which the Company could incur any liability under ERISA
(collectively, the "Benefit Plans"), no event has occurred and, to the best
knowledge of the Company, there exists no condition or set of circumstances, in
connection with which the Company could be subject to any liability under the
terms of such Benefit Plan, applicable law (including, without limitation, ERISA
and the Internal Revenue Code of 1986, as amended) or any applicable agreement
that could have a Material Adverse Effect.

     4.   Agreements of the Company.  The Company agrees with you as follows:

          (a) The Company will not, during such period as the Prospectus is
required by law to be delivered in connection with sales of the Securities by
you or a dealer, file any amendment or supplement to the Registration Statement
or the Prospectus, unless a copy thereof shall first have been submitted to you
within a reasonable period of time prior to the filing thereof and you shall not
have objected thereto in good faith.

          (b) The Company will notify you promptly, and will confirm such advice
in writing, (i) when any post-effective amendment to the Registration Statement
becomes effective, (ii) of any request by the Commission for amendments or
supplements to the Registration Statement or the Prospectus or for additional
information, (iii) of the issuance by the Commission of any stop order
suspending the effectiveness of the Registration Statement or the initiation of
any proceedings for that purpose or the threat thereof, (iv) of the happening of
any event during the period mentioned in the second sentence of Section 4(e)
that in the judgment of the Company makes any statement made in the Registration
Statement or the Prospectus untrue or that requires the making of any changes in
the Registration Statement or the Prospectus in order to make the statements
therein, in light of the circumstances in which they are made, not misleading
and (v) of receipt by the Company or any representative or attorney of the
Company of any other communication from the Commission relating to the Company,
the Registration Statement, any preliminary prospectus or the Prospectus.  If at
any time the Commission shall issue any order suspending the effectiveness of
the Registration Statement, the Company will make every reasonable effort to
obtain the withdrawal of such order at the earliest possible moment.  The
Company will use its best efforts to comply with the provisions of and make all
requisite filings with the Commission pursuant to Rule 430A and to notify you
promptly of all such filings.

          (c) The Company has furnished, or will furnish, to you, without
charge, two conformed copies of the Registration Statement and of any post-
effective amendment thereto, 

                                       9
<PAGE>
 
including financial statements and schedules, and all exhibits thereto
(including any document filed under the Exchange Act and deemed to be
incorporated by reference into the Prospectus).

          (d) The Company will comply with all the provisions of any
undertakings contained in the Registration Statement.

          (e) On the Effective Date, and thereafter from time to time, the
Company will deliver to the Underwriter, without charge, as many copies of the
Prospectus or any amendment or supplement thereto, as you may reasonably
request.  The Company consents to the use of the Prospectus or any amendment or
supplement thereto by the Underwriter and by all dealers to whom the Securities
may be sold, both in connection with the offering or sale of the Securities and
for any period of time thereafter during which the Prospectus is required by law
to be delivered in connection therewith.  If during such period of time any
event shall occur which in the judgment of the Company or counsel to the
Underwriter should be set forth in the Prospectus in order to make any statement
therein, in the light of the circumstances under which it was made, not
misleading, or if it is necessary to supplement or amend the Prospectus to
comply with law, the Company will forthwith prepare and duly file with the
Commission an appropriate supplement or amendment thereto, and will deliver to
the Underwriter, without charge, such number of copies thereof as you may
reasonably request.  The Company shall not file any document under the Exchange
Act before the termination of the offering of the Securities by you if such
document would be deemed to be incorporated by reference into the Prospectus and
if such document is not approved by the Underwriter after reasonable notice
thereof.

          (f) Prior to any public offering of the Securities by the Underwriter,
the Company will cooperate with the Underwriter and the Underwriter's counsel in
connection with the registration or qualification of the Securities for offer
and sale under the securities or Blue Sky laws of such jurisdictions as you may
request; provided, that in no event shall the Company be obligated to qualify to
do business in any jurisdiction where it is not now so qualified or to take any
action which would subject it to general service of process in any jurisdiction
where it is not now so subject.

          (g) During the period of five years commencing on the date of the
Price Determination Agreement, the Company will furnish to you copies of such
financial statements and other periodic and special reports as the Company may
from time to time distribute generally to the holders of any class of its
capital stock, and will furnish to you a copy of each annual or other report it
shall be required to file with the Commission.

          (h) The Company will make generally available to holders of its
securities as soon as may be practicable but in no event later than the last day
of the fifteenth full calendar month following the calendar quarter in which the
"effective date of the Registration Statement" (as defined in Rule 158 of the
Rules and Regulations) falls, an earnings statement (which need not be audited
but shall be in reasonable detail) for a period of 12 months ended commencing
after such "effective date of the Registration Statement," and satisfying the
provisions of Section 11(a) of the Act (including Rule 158 of the Rules and
Regulations).

          (i) Whether or not the transactions contemplated by this Agreement are
consummated or this Agreement is terminated, the Company will pay, or reimburse
if paid by you, all costs and expenses incident to the performance of the
obligations of the Company under 

                                       10
<PAGE>
 
this Agreement, including but not limited to costs and expenses of or relating
to (i) the preparation, printing and filing of the Registration Statement and
exhibits to it, each preliminary prospectus, the Prospectus, any amendment or
supplement to the Registration Statement or the Prospectus and the Indenture,
(ii) the preparation and delivery of certificates representing the Securities,
(iii) the printing of this Agreement and any Dealer Agreements, (iv) furnishing
(including costs of shipping, mailing and courier) such copies of the
Registration Statement, the Prospectus and any preliminary prospectus, and all
amendments and supplements thereto, as may be requested for use in connection
with the offering and sale of the Securities by the Underwriters or by dealers
to whom Securities may be sold, (v) any filings required to be made by the
Underwriter with the NASD, and the fees, disbursements and other charges of
counsel for the Underwriter in connection therewith, (vi) the registration or
qualification of the Securities for offer and sale under the securities or Blue
Sky laws of such jurisdictions designated pursuant to Section 4(f), including
the fees, disbursements and other charges of counsel to the Underwriter in
connection therewith, and the preparation and printing of preliminary,
supplemental and final Blue Sky memoranda, (vii) counsel to the Company, (viii)
the transfer agent and registrar for the Securities, (ix) the rating of the
Securities by one or more rating agencies, (x) the Trustee and any agent of the
Trustee and the fees, disbursements and other charges of counsel for the Trustee
in connection with the Indenture and the Securities and (xi) the Accountants.
Except as otherwise provided in this Section 4, the Underwriter shall pay all of
its own costs and expenses.

          (j) If this Agreement shall be terminated by the Company pursuant to
any of the provisions hereof or if for any reason the Company shall be unable to
perform its obligations hereunder, the Company will reimburse the Underwriter
for all out- of-pocket expenses (including the fees, disbursements and other
charges of counsel to the Underwriter) reasonably incurred by it in connection
herewith.

          (k) The Company will not at any time, directly or indirectly, take any
action intended, or which might reasonably be expected, to cause or result in,
or which will constitute, stabilization of the price of the Securities to
facilitate the sale or resale of any of the Securities.

          (l) The Company will apply the net proceeds from the offering and sale
of the Securities in the manner set forth in the Prospectus under "Use of
Proceeds."

          (m) The Company will not claim the benefit of any usury law against
any holders of Securities.

     5.   Conditions of Obligations of the Underwriters.  In addition to the
execution and delivery of the Price Determination Agreement, the obligations of
the Underwriter hereunder are subject to the following conditions:

          (a) (i) No stop order suspending the effectiveness of the Registration
Statement shall have been issued and no proceedings for that purpose shall be
pending or threatened by the Commission, (ii) no order suspending the
effectiveness of the Registration Statement or the qualification or registration
of the Securities under the securities or Blue Sky laws of any jurisdiction
shall be in effect and no proceeding for such purpose shall be pending before or
threatened or contemplated by the Commission or the authorities of any such
jurisdiction, (iii) any request for additional information on the part of the
staff of the Commission or any such authorities shall have been complied with to
the satisfaction of the staff of the

                                       11
<PAGE>
 
Commission or such authorities and (iv) after the date hereof no amendment or
supplement to the Registration Statement or the Prospectus shall have been filed
unless a copy thereof was first submitted to you and you did not object thereto
in good faith, and you shall have received certificates, dated the Closing Date
and signed by the Chief Executive Officer, President or a Vice President of the
Company and the Chief Financial Officer of the Company (who may, as to
proceedings threatened, rely upon the best of their information and belief), to
the effect of clauses (i), (ii) and (iii).

          (b) Since the respective dates as of which information is given in the
Registration Statement and the Prospectus (i) there shall not have been a
material adverse change in the general affairs, business, business prospects,
properties, management, condition (financial or otherwise) or results of
operations of the Company and its Subsidiaries, taken as a whole, whether or not
arising from transactions in the ordinary course of business, in each case other
than as set forth in or contemplated by the Registration Statement and the
Prospectus, and (ii) neither the Company nor any of its Subsidiaries shall have
sustained any material loss or interference with its business or properties from
fire, explosion, flood or other casualty, whether or not covered by insurance,
or from any labor dispute or any court or legislative or other governmental
action, order or decree, which is not set forth in the Registration Statement
and the Prospectus, if in your judgment any such development makes it
impracticable or inadvisable to consummate the sale and delivery of the
Securities by the Underwriter in accordance with the terms hereof and thereof.

          (c) Since the respective dates as of which information is given in the
Registration Statement and the Prospectus, there shall have been no litigation
or other proceeding instituted against the Company or any of its Subsidiaries or
any of their respective officers or directors in their capacities as such,
before or by any Federal, state or local court, commission, regulatory body,
administrative agency or other governmental body, domestic or foreign, in which
litigation or proceeding an unfavorable ruling, decision or finding would
materially and adversely affect the business, properties, business prospects,
condition (financial or otherwise) or results of operations of the Company and
its Subsidiaries taken as a whole.

          (d) Each of the representations and warranties of the Company
contained herein shall be true and correct in all material respects at the
Closing Date, as if made and all covenants and agreements herein contained to be
performed on the part of the Company and all conditions herein contained to be
fulfilled or complied with by the Company at or prior to the Closing Date shall
have been duly performed, fulfilled or complied with.

          (e) The Underwriter shall have received opinions, each dated the
Closing Date and satisfactory in form and substance to counsel for you, from
Gable & Gotwals, counsel to the Company, to the effect set forth in Exhibit B
and Anderson, Byrd, Richeson, Flaherty and Henrichs, special counsel to the
Company.

          (f) The Underwriter shall have received an opinion, dated the Closing
Date from Jones, Day, Reavis & Pogue, counsel to the Underwriter, with respect
to the Registration Statement, the Prospectus and this Agreement, which opinion
shall be satisfactory in all respects to you.

                                       12
<PAGE>
 
          (g) On the date of the Price Determination Agreement, the Accountants
shall have furnished to the Underwriter letters, each dated the date of its
delivery, addressed to the Underwriter and in form and substance satisfactory to
the Underwriter, confirming that they are independent accountants with respect
to the Company or Southwest Gas, as the case may be, as required by the Act and
the Rules and Regulations and with respect to the financial and other
statistical and numerical information contained or incorporated by reference in
the Registration Statement.  At the Closing Date, the Accountants shall have
furnished to the Underwriter letters, each dated the date of its delivery, which
shall confirm, on the basis of a review in accordance with the procedures set
forth in the respective letters from the Accountants, that nothing has come to
their attention during the period from the date of the letter referred to in the
prior sentence to a date (specified in the letter) not more than five days prior
to the Closing Date which would require any change in their letter dated the
date of the Price Determination Agreement, if it were required to be dated and
delivered at the Closing Date.

          (h) At the Closing Date, there shall be furnished to the Underwriter
an accurate certificate, dated the date of its delivery, signed by each of the
Chief Executive Officer, President or a Vice President and the Chief Financial
Officer of the Company, in form and substance satisfactory to you, to the effect
that:

               (i) Each signer of such certificate has carefully examined the
Registration Statement and the Prospectus (including any documents filed under
the Exchange Act and deemed to be incorporated by reference into the Prospectus)
and (A) as of the date of such certificate, such documents are true and correct
in all material respects and do not omit to state a material fact required to be
stated therein or necessary in order to make the statements therein not untrue
or misleading and (B) since the Effective Date, no event has occurred as a
result of which it is necessary to amend or supplement the Prospectus in order
to make the statements therein not untrue or misleading in any material respect
and there has been no document required to be filed under the Exchange Act and
the Exchange Act Rules and Regulations that upon such filing would be deemed to
be incorporated by reference into the Prospectus that has not been so filed.

               (ii) Each of the representations and warranties of the Company
contained in this Agreement were, when originally made, and are, at the time
such certificate is delivered, true and correct in all material respects.

               (iii) Each of the covenants required herein to be performed by
the Company on or prior to the delivery of such certificate has been duly,
timely and fully performed and each condition herein required to be complied
with by the Company on or prior to the date of such certificate has been duly,
timely and fully complied with.

          (i) The Securities shall be qualified for sale in such states as the
Underwriter may reasonably request, each such qualification shall be in effect
and not subject to any stop order or other proceeding on the Closing Date.

          (j) Subsequent to the execution and delivery of this Agreement and
prior to the Closing Date, there shall not have been any downgrading, nor any
notice given of any intended or potential downgrading or of a possible change
that does not indicate the direction of the possible change, in the rating
accorded any of the Company's securities, including the 

                                       13
<PAGE>
 
Securities, by any "nationally recognized statistical rating organization," as
such term is defined for purposes of Rule 436(g)(2) under the 1933 Act.

          (k) The Company shall have furnished to the Underwriter such
certificates, in addition to those specifically mentioned herein, as the
Underwriter may have reasonably requested as to the accuracy and completeness at
the Closing Date of any statement in the Registration Statement or the
Prospectus or any documents filed under the Exchange Act and deemed to be
incorporated by reference into the Prospectus, as to the accuracy at the Closing
Date of the representations and warranties of the Company herein, as to the
performance by the Company of its obligations hereunder, or as to the
fulfillment of the conditions concurrent and precedent to the obligations
hereunder of the Underwriter.

     6.   Indemnification.

          (a) The Company will indemnify and hold harmless the Underwriter, the
directors, officers, employees and agents of the Underwriter and each person, if
any, who controls the Underwriter within the meaning of Section 15 of the Act or
Section 20 of the Exchange Act from and against any and all losses, claims,
liabilities, expenses and damages (including, but not limited to, any and all
investigative, legal and other expenses reasonably incurred in connection with,
and any and all amounts paid in settlement of, any action, suit or proceeding
between any of the indemnified parties and any indemnifying parties or between
any indemnified party and any third party, or otherwise, or any claim asserted),
as and when incurred, to which the Underwriter, or any such person, may become
subject under the Act, the Exchange Act or other Federal or state statutory law
or regulation, at common law or otherwise, insofar as such losses, claims,
liabilities, expenses or damages arise out of or are based on (i) any untrue
statement or alleged untrue statement of a material fact contained in any
preliminary prospectus, the Registration Statement or the Prospectus or any
amendment or supplement to the Registration Statement or the Prospectus or in
any documents filed under the Exchange Act and deemed to be incorporated by
reference into the Prospectus by or on behalf of the Company or based on written
information furnished by or on behalf of the Company filed in any jurisdiction
in order to qualify the Securities under the securities laws thereof or filed
with the Commission, (ii) the omission or alleged omission to state in such
document a material fact required to be stated in it or necessary to make the
statements in it not misleading or (iii) any act or failure to act or any
alleged act or failure to act by the Underwriter in connection with, or relating
in any manner to, the Securities or the offering contemplated hereby, and which
is included as part of or referred to in any loss, claim, liability, expense or
damage arising out of or based upon matters covered by clause (i) or (ii) above
(provided that the Company shall not be liable under this clause (iii) to the
extent it is finally judicially determined by a court of competent jurisdiction
that such loss, claim, liability, expense or damage resulted directly from any
such acts or failures to act undertaken or omitted to be taken by such
underwriter through its gross negligence or willful misconduct); provided that
the Company will not be liable to the extent that such loss, claim, liability,
expense or damage arises from the sale of the Securities in the public offering
to any person by the Underwriter and is based on an untrue statement or omission
or alleged untrue statement or omission made in reliance on and in conformity
with information relating to the Underwriter furnished in writing to the Company
by the Underwriter expressly for inclusion in the Registration Statement, any
preliminary prospectus or the Prospectus.  This indemnity agreement will be in
addition to any liability that the Company might otherwise have.

                                       14
<PAGE>
 
          (b) The Underwriter will indemnify and hold harmless the Company, each
person, if any, who controls the Company within the meaning of Section 15 of the
Act or Section 20 of the Exchange Act, each director of the Company and each
officer of the Company who signs the Registration Statement to the same extent
as the foregoing indemnity from the Company to the Underwriter, but only insofar
as losses, claims, liabilities, expenses or damages arise out of or are based on
any untrue statement or omission or alleged untrue statement or omission made in
reliance on and in conformity with information relating to the Underwriter
furnished in writing to the Company by the Underwriter expressly for use in the
Registration Statement, any preliminary prospectus or the Prospectus.  This
indemnity agreement will be in addition to any liability that the Underwriter
might otherwise have; provided, however, that in no case shall the Underwriter
be liable or responsible for any amount in excess of the underwriting discounts
and commissions received by the Underwriter.

          (c) Any party that proposes to assert the right to be indemnified
under this Section 6 will, promptly after receipt of notice of commencement of
any action against such party in respect of which a claim is to be made against
an indemnifying party or parties under this Section 6, notify each such
indemnifying party of the commencement of such action, enclosing a copy of all
papers served, but the omission so to notify such indemnifying party will not
relieve it from any liability that it may have to any indemnified party under
the foregoing provisions of this Section 6 unless, and only to the extent that,
such omission results in the forfeiture of substantive rights or defenses by the
indemnifying party.  If any such action is brought against any indemnified party
and it notifies the indemnifying party of its commencement, the indemnifying
party will be entitled to participate in and, to the extent that it elects by
delivering written notice to the indemnified party promptly after receiving
notice of the commencement of the action from the indemnified party to assume
the defense of the action, with counsel satisfactory to the indemnified party,
and after notice from the indemnifying party to the indemnified party of its
election to assume the defense, the indemnifying party will not be liable to the
indemnified party for any legal or other expenses except as provided below and
except for the reasonable costs of investigation subsequently incurred by the
indemnified party in connection with the defense.  The indemnified party will
have the right to employ its own counsel in any such action, but the fees,
expenses and other charges of such counsel will be at the expense of such
indemnified party unless (i) the employment of counsel by the indemnified party
has been authorized in writing by the indemnifying party, (ii) the indemnified
party has reasonably concluded (based on advice of counsel) that there may be
legal defenses available to it or other indemnified parties that are different
from or in addition to those available to the indemnifying party, (iii) a
conflict or potential conflict exists (based on advice of counsel to the
indemnified party) between the indemnified party and the indemnifying party (in
which case the indemnifying party will not have the right to direct the defense
of such action on behalf of the indemnified party) or (iv) the indemnifying
party has not in fact employed counsel to assume the defense of such action
within a reasonable time after receiving notice of the commencement of the
action, in each of which cases the reasonable fees, disbursements and other
charges of counsel will be at the expense of the indemnifying party or parties.
It is understood that the indemnifying party shall not, in connection with any
proceeding or related proceedings in the same jurisdiction, be liable for the
reasonable fees, disbursements and other charges of more than one separate firm
admitted to practice in such jurisdiction at any one time for all such
indemnified party or parties.  All such fees, disbursements and other charges
will be reimbursed by the indemnifying party promptly as they are incurred.  An
indemnifying party will not be liable for any settlement of any action or claim
effected without its written consent (which 

                                       15
 
<PAGE>
 
consent will not be unreasonably withheld). No indemnifying party shall, without
the prior written consent of each indemnified party, settle or compromise or
consent to the entry of any judgment in any pending or threatened claim, action
or proceeding relating to the matters contemplated by this Section 6 (whether or
not any indemnified party is a party thereto), unless such settlement,
compromise or consent includes an unconditional release of each indemnified
party from all liability arising or that may arise out of such claim, action or
proceeding. Notwithstanding any other provision of this Section 6(c), if at any
time an indemnified party shall have requested an indemnifying party to
reimburse the indemnified party for fees and expenses of counsel, such
indemnifying party agrees that it shall be liable for any settlement effected
without its written consent if (1) such settlement is entered into more than 45
days after receipt by such indemnifying party of the aforesaid request, (2) such
indemnifying party shall have received notice of the terms of such settlement at
least 30 days prior to such settlement being entered into and (3) such
indemnifying party shall not have reimbursed such indemnified party in
accordance with such request prior to the date of such settlement.

          (d) In order to provide for just and equitable contribution in
circumstances in which the indemnification provided for in the foregoing
paragraphs of this Section 6 is applicable in accordance with its terms but for
any reason is held to be unavailable from the Company or the Underwriter, the
Company and the Underwriter will contribute to the total losses, claims,
liabilities, expenses and damages (including any investigative, legal and other
expenses reasonably incurred in connection with, and any amount paid in
settlement of, any action, suit or proceeding or any claim asserted, but after
deducting any contribution received by the Company from persons other than the
Underwriter, such as persons who control the Company within the meaning of the
Act, officers of the Company who signed the Registration Statement and directors
of the Company, who also may be liable for contribution) to which the Company
and the Underwriter may be subject in such proportion as shall be appropriate to
reflect the relative benefits received by the Company on the one hand and the
Underwriter on the other.  The relative benefits received by the Company on the
one hand and the Underwriter on the other shall be deemed to be in the same
proportion as the total net proceeds from the offering (before deducting
expenses) received by the Company bear to the total underwriting discounts and
commissions received by the Underwriter, in each case as described in the
Prospectus.  If, but only if, the allocation provided by the foregoing sentence
is not permitted by applicable law, the allocation of contribution shall be made
in such proportion as is appropriate to reflect not only the relative benefits
referred to in the foregoing sentence but also the relative fault of the
Company, on the one hand, and the Underwriter, on the other, with respect to the
statements or omissions which resulted in such loss, claim, liability, expense
or damage, or action in respect thereof, as well as any other relevant equitable
considerations with respect to such offering.  Such relative fault shall be
determined by reference to whether the untrue or alleged untrue statement of a
material fact or omission or alleged omission to state a material fact relates
to information supplied by the Company or the Underwriter, the intent of the
parties and their relative knowledge, access to information and opportunity to
correct or prevent such statement or omission.  The Company and the Underwriter
agree that it would not be just and equitable if contributions pursuant to this
Section 6(d) were to be determined by pro rata allocation or by any other method
of allocation which does not take into account the equitable considerations
referred to herein.  The amount paid or payable by an indemnified party as a
result of the loss, claim, liability, expense or damage, or action in respect
thereof, referred to above in this Section 6(d) shall be deemed to include, for
purpose of this Section 6(d), any legal or other expenses reasonably incurred by
such indemnified party in connection with investigating or defending any 

                                       16
<PAGE>
 
such action or claim. Notwithstanding the provisions of this Section 6(d), the
Underwriter shall not be required to contribute any amount in excess of the
underwriting discounts and commissions received by it and no person found guilty
of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act)
will be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation. For purposes of this Section 6(d), any person who
controls a party to this Agreement within the meaning of the Act will have the
same rights to contribution as that party, and each officer of the Company who
signed the Registration Statement will have the same rights to contribution as
the Company, subject in each case to the provisions hereof. Any party entitled
to contribution, promptly after receipt of notice of commencement of any action
against such party in respect of which a claim for contribution may be made
under this Section 6(d), will notify any such party or parties from whom
contribution may be sought, but the omission so to notify will not relieve the
party or parties from whom contribution may be sought from any other obligation
it or they may have under this Section 6(d). Except for a settlement entered
into pursuant to the last sentence of Section 6(c) hereof, no party will be
liable for contribution with respect to any action or claim settled without its
written consent (which consent will not be unreasonably withheld).

          (e) The indemnity and contribution agreements contained in this
Section 6 and the representations and warranties of the Company contained in
this Agreement shall remain operative and in full force and effect regardless of
(i) any investigation made by or on behalf of the Underwriter, (ii) acceptance
of any of the Securities and payment therefor or (iii) any termination of this
Agreement.

     7.   Termination.  The obligations of the Underwriter under this Agreement
may be terminated at any time on or prior to the Closing Date by notice to the
Company from you, without liability on the part of the Underwriter to the
Company, if, prior to delivery and payment for the Securities in the sole
judgment of the Underwriter, (a) trading in securities generally on the New York
Stock Exchange shall have been suspended or limited or minimum or maximum prices
shall have been generally established on such exchange, or additional material
governmental restrictions, not in force on the date of this Agreement, shall
have been imposed upon trading in securities generally by such exchange or by
order of the Commission or any court or other governmental authority, (b) a
general banking moratorium shall have been declared by either Federal or New
York State authorities or (c) any material adverse change in the financial or
securities markets in the United States or in political, financial or economic
conditions in the United States or any outbreak or material escalation of
hostilities or declaration by the United States of a national emergency or war
or other calamity or crisis shall have occurred the effect of any of which is
such as to make it, in the sole judgment of the Underwriter, impracticable or
inadvisable to market the Securities on the terms and in the manner contemplated
by the Prospectus.

     8.   Miscellaneous.  Notice given pursuant to any of the provisions of this
Agreement shall be in writing and, unless otherwise specified, shall be mailed
or delivered (a) if to the Company, at the office of the Company, 100 West Fifth
Street, Tulsa, Oklahoma 74103, Attention: Chief Financial Officer or (b) if to
the Underwriter at ____________________, Attention:  ________________.  Any such
notice shall be effective only upon receipt.  Any notice under Section 7 may be
made by telex or telephone, but if so made shall be subsequently confirmed in
writing.

                                       17
<PAGE>
 
     This Agreement has been and is made solely for the benefit of the
Underwriter and the Company and of the controlling persons, directors and
officers referred to in Section 6, and their respective successors and assigns,
and no other person shall acquire or have any right under or by virtue of this
Agreement.  The term "successors and assigns" as used in this Agreement shall
not include a purchaser, as such purchaser, of Securities from the Underwriter.

     All representations, warranties and agreements of the Company contained
herein or in certificates or other instruments delivered pursuant hereto, shall
remain operative and in full force and effect regardless of any investigation
made by or on behalf of the Underwriter or any of its controlling persons and
shall survive delivery of and payment for the Securities hereunder.

     THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO THE CONFLICT OF LAWS PRINCIPLES
OF SUCH STATE.

     This Agreement may be signed in two or more counterparts with the same
effect as if the signatures thereto and hereto were upon the same instrument.
In case any provision in this Agreement shall be invalid, illegal or
unenforceable, the validity, legality and enforceability of the remaining
provisions shall not in any way be affected or impaired thereby.

     The Company and the Underwriter each hereby irrevocably waive any right
they may have to trial by jury in respect of any claim based upon or arising out
of this Agreement or the transactions contemplated hereby.

     This Agreement may not be amended or otherwise modified or any provision
hereof waived except by an instrument in writing signed by the Underwriter and
the Company.


                            [signature page follows]

                                       18
 
<PAGE>
 
     Please confirm that the foregoing correctly sets forth the agreement
between the Company and the Underwriter.

                                        Very truly yours,

                                        ONEOK, INC.


                                        By:
                                           ---------------------------------
                                        Title:
 


Confirmed as of the date first
above mentioned:


- ------------------------------


By:
   --------------------------------
Title:

                                       19
<PAGE>
 
                                                                       EXHIBIT A



                                  ONEOK, INC.



                         PRICE DETERMINATION AGREEMENT


                                                              ____________, 1999



- --------------------



Dear Sirs:

     Reference is made to the Underwriting Agreement, dated ___________, 1999
(the "Underwriting Agreement"), between ONEOK, Inc., an Oklahoma corporation
(the "Company"), and ________________ ("you" or the "Underwriter").  The
Underwriting Agreement provides for the purchase by the Underwriter from the
Company, subject to the terms and conditions set forth therein, of an aggregate
of $___________ principal amount of the Company's Debt Securities (the
"Securities") to be issued pursuant to an Indenture dated as of September 24,
1998 between the Company and Chase Bank of Texas, National Association, as
Trustee.  This Agreement is the Price Determination Agreement referred to in the
Underwriting Agreement.

     Pursuant to Section 1 of the Underwriting Agreement, the undersigned agree
with you that the purchase price for the Securities to be paid by you shall be
____% of the aggregate principal amount of the Securities.

     The Company represents and warrants to you that the representations and
warranties of the Company set forth in Section 3 of the Underwriting Agreement
are accurate as though expressly made at and as of the date hereof.

     This Agreement shall be governed by the law of the State of New York
without regard to the conflict of law principles of such State.

     If the foregoing is in accordance with your understanding of the agreement
between the Underwriter and the Company, please sign and return to the Company a
counterpart hereof, whereupon this instrument along with all counterparts and
together with the Underwriting 

                                      A-1
<PAGE>
 
Agreement shall be a binding agreement between the Underwriter and the Company
in accordance with its terms and the terms of the Underwriting Agreement.

                                      Very truly yours,



                                      ONEOK, INC.


                                      By:
                                         -----------------------------------
                                      Title:



Confirmed as of the date
  first above mentioned:


- -------------------------


By:
   --------------------------------
Title:

                                      A-2
<PAGE>
 
                                                                       EXHIBIT B



                               Form of Opinion of
                             Counsel to the Company


     1.   The Company and each of its Subsidiaries is a corporation or limited
liability company duly organized, validly existing and in good standing under
the laws of the jurisdiction of its incorporation or organization and has full
corporate power and authority to conduct all the activities conducted by it, to
own or lease all the assets owned or leased by it and to conduct its business as
described in the Registration Statement and the Prospectus.  The Company is the
sole record owner and, to our knowledge, the sole beneficial owner of all of the
capital stock or capital interests of each of its Subsidiaries.

     2.   No consent, approval, authorization or order of, or any filing or
declaration with, any court or governmental agency or body is required in
connection with the authorization, issuance, transfer, sale or delivery of the
Securities, in connection with the execution, delivery and performance of the
Agreement by the Company or in connection with the taking by the Company of any
action contemplated thereby except such as have been obtained under the Act, the
Trust Indenture Act and the Rules and Regulations and such as may be required
under state securities or "Blue Sky" laws or by the by-laws and rules of the
NASD in connection with the purchase and distribution by the Underwriters of the
Securities.

     3.   The Registration Statement and the Prospectus (including any documents
incorporated by reference into the Prospectus, at the time they were filed)
comply or complied in all material respects as to form with the requirements of
the Act, the Rules and Regulations, the Exchange Act, the Exchange Act Rules and
Regulations, the Trust Indenture Act and the Trust Indenture Act Rules and
Regulations and the Indenture complies in all material respects as to form with
the Trust Indenture Act (except that such counsel need not express an opinion as
to (a) financial statements, schedules and other financial data contained in the
Registration Statement or the Prospectus (or incorporated by reference therein)
and (b) the Statement of Eligibility and Qualification under the Trust Indenture
Act of the Trustee on Form T-1).

     4.   To the best of such counsel's knowledge, any instrument, document,
lease, license, contract or other agreement (collectively, "Documents") required
to be described or referred to in the Registration Statement or the Prospectus
has been properly described or referred to therein and any Document required to
be filed as an exhibit to the Registration Statement has been filed as an
exhibit thereto or has been incorporated as an exhibit by reference in the
Registration Statement; and no default exists in the due performance or
observance of any material obligation, agreement, covenant or condition
contained in any Document filed or required to be filed as an exhibit to the
Registration Statement.

     5.   To the best of such counsel's knowledge, no person or entity has the
right to require the registration under the Act of shares of common stock or
other securities of the Company by reason of the filing or effectiveness of the
Registration Statement.

                                      B-1
<PAGE>
 
     6.   To the best of such counsel's knowledge, the Company is not in
violation of, or in default with respect to, any law, rule, regulation, order,
judgment or decree, except as may be described in the Prospectus or such as in
the aggregate do not now have and will not in the future have a Material Adverse
Effect.

     7.   The Securities and the Indenture conform in all material respects as
to legal matters to the description thereof contained in the Registration
Statement and the Prospectus.  All descriptions in the Prospectus of statutes,
regulations or legal or governmental proceedings are accurate and fairly present
the information required to be shown in all material respects.

     8.   The Company has full corporate power and authority to enter into the
Agreement, and the Agreement has been duly authorized, executed and delivered by
the Company, is a valid and binding agreement of the Company and, except for the
indemnification and contribution provisions thereof, as to which such counsel
need not express an opinion, is enforceable against the Company in accordance
with the terms thereof, subject to bankruptcy, insolvency, reorganization,
fraudulent transfer or conveyance or other laws of general applicability
relating to or affecting creditors' rights and to general principles of equity,
whether such principles are considered in a proceeding at law or equity.

     9.   The Company has full corporate power and authority to enter into the
Indenture and to issue the Securities, and the Indenture has been duly
authorized, executed and delivered by the Company and duly qualified under the
Trust Indenture Act; the Securities have been duly authorized, executed,
authenticated (assuming due authentication by or on behalf of the Trustee),
issued and delivered and are entitled to the benefits of the Indenture; and,
subject to bankruptcy, insolvency, reorganization, fraudulent transfer or
conveyance or other laws of general applicability relating to or affecting
creditors' rights and to general principles of equity, whether such principles
are considered in a proceeding at law or equity, each of the Indenture and the
Securities are legal, valid and binding obligations of the Company and are
enforceable against the Company in accordance with their respective terms.

     10.  The execution and delivery by the Company of, and the performance by
the Company of its agreements in, the Agreement, the Indenture and the
Securities do not and will not (a) violate the certificate of incorporation or
by-laws of the Company, (b) breach or result in a default under, cause the time
for performance of any obligation to be accelerated under, or result in the
creation or imposition of any lien, charge or encumbrance upon any of the assets
of the Company or any of its Subsidiaries pursuant to the terms of, (i) any
indenture, mortgage, deed of trust, loan agreement, bond, debenture, note
agreement, capital lease or other evidence of indebtedness of which such counsel
has knowledge, (ii) any voting trust arrangement or any contract or other
agreement to which the Company is a party that restricts the ability of the
Company to issue securities or (iii) any Document filed as an exhibit to, or
incorporated as an exhibit by reference in, the Registration Statement, (c)
breach or otherwise violate any existing obligation of the Company under any
court or administrative order, judgment or decree of which such counsel has
knowledge or (d) violate applicable provisions of any statute or regulation in
the State of Oklahoma or of the United States.

          11.  The Company is not an "investment company" or an "affiliated
person" of, or "promoter" or "principal underwriter" for, an "investment
company," as such terms are defined in the Investment Company Act of 1940, as
amended.

                                      B-2
<PAGE>
 
     Such counsel hereby confirms to the Underwriter that such counsel has been
advised by the Commission that the Registration Statement has become effective
under the Act and that no order suspending the effectiveness of the Registration
Statement has been issued and no proceeding for that purpose has been instituted
or is threatened, pending or contemplated.

     Such counsel hereby further confirms to the Underwriter that there are no
actions, suits, proceedings or investigations pending or, to such counsel's
knowledge, overtly threatened in writing against the Company or any of its
Subsidiaries, or any of their respective officers or directors in their
capacities as such, before or by any court, governmental agency or arbitrator
which (i) seek to challenge the legality or enforceability of the Agreement, the
Indenture or the Securities, (ii) seek to challenge the legality or
enforceability of any of the Documents filed, or required to be filed, as
exhibits to the Registration Statement, (iii) seek damages or other remedies
with respect to any of the Documents filed, or required to be filed, as exhibits
to the Registration Statement, (iv) except as set forth in or contemplated by
the Registration Statement and the Prospectus, seek money damages in excess of
$1,000,000 or seek to impose criminal penalties upon the Company, any of its
Subsidiaries or any of their respective officers or directors in their
capacities as such and of which such counsel has knowledge or (v) seek to enjoin
any of the business activities of the Company or any of its Subsidiaries or the
transactions described in the Prospectus and of which such counsel has
knowledge.

     Such counsel has participated in the preparation of the Registration
Statement and the Prospectus and, without assuming any responsibility for the
accuracy, completeness or fairness of the statements contained in the
Registration Statement or the Prospectus or in any amendment or supplement
thereto or in any document incorporated by reference into the Prospectus,
nothing has come to such counsel's attention that causes such counsel to believe
that, both as of the Effective Date and as of the Closing Date, the Registration
Statement, or any amendment thereto, contained or contains any untrue statement
of a material fact or omitted or omits to state a material fact required to be
stated therein or necessary to make the statements therein not misleading or
that any Prospectus or any amendment or supplement thereto including any
documents incorporated by reference into the Prospectus, at the time such
Prospectus was issued, at the time any such amended or supplemented Prospectus
was issued, at the Closing Date, contained or contains any untrue statement of a
material fact or omitted or omits to state a material fact necessary in order to
make the statements therein, in the light of the circumstances in which they
were made, not misleading (except that such counsel need not express an opinion
as to (a) financial statements, schedules and other financial data contained in
the Registration Statement or the Prospectus (or incorporated by reference
therein) and (b) the Statement of Eligibility and Qualification under the Trust
Indenture Act of the Trustee on Form T-1).

     In rendering the foregoing opinion, counsel may rely, to the extent they
deem such reliance proper, on the opinions of their counsel as to matters
governed by the laws of jurisdictions other than the United States and the State
of Oklahoma, and as to matters of fact, upon certificates of officers of the
Company and of government officials; provided that such counsel shall state that
in such counsel's opinion that such counsel and the Underwriter are justified in
relying on such opinions of other counsel.  Copies of all such opinions and
certificates shall be furnished to counsel to the Underwriter on the Closing
Date.  For purposes of paragraph 2 and 10(d) above, such counsel has reviewed
only those statutes, rules and regulations, including regulatory rules and
regulations, that in such counsel's experience are 

                                      B-3
<PAGE>
 
applicable to transactions of the type contemplated by the Agreement or the
Indenture or for the offering, issuance, sale or delivery of the Securities.

     Capitalized terms used in such opinion but not defined shall have the
meanings assigned to them in the Agreement.

                                      B-4

<PAGE>
 
                                                                     Exhibit 4.5


================================================================================


                                  ONEOK, INC.

                      6.40% SENIOR INSURED QUARTERLY NOTES

                              Due February 1, 2019

                                 ------------

                         FOURTH SUPPLEMENTAL INDENTURE

                         Dated as of February 17, 1999

                                 ------------

                   Chase Bank of Texas, National Association

                                    TRUSTEE


================================================================================
<PAGE>
 
          THIS FOURTH SUPPLEMENTAL INDENTURE is made as of the seventeenth day
of February, 1999, by and between ONEOK, Inc., an Oklahoma corporation (the
"Company"), and CHASE BANK OF TEXAS, NATIONAL ASSOCIATION, a national banking
association (the "Trustee").

                             W I T N E S S E T H :

          WHEREAS, the Company has heretofore entered into an Indenture, dated
as of September 24, 1998 (the "Original Indenture"), with the Trustee;

          WHEREAS, the Original Indenture is incorporated herein by this
reference and the Original Indenture, as supplemented by this Fourth
Supplemental Indenture, is herein called the "Indenture";

          WHEREAS, under the Original Indenture, a new series of Securities may
at any time be established pursuant to a supplemental indenture executed by the
Company and the Trustee;

          WHEREAS, the Company proposes to create under the Indenture a new
series of Securities; and

          WHEREAS, all conditions necessary to authorize the execution and
delivery of this Fourth Supplemental Indenture and to make it a valid and
binding obligation of the Company have been done or performed.

          NOW, THEREFORE, in consideration of the agreements and obligations set
forth herein and for other good and valuable consideration, the sufficiency of
which is hereby acknowledged, the parties hereto hereby agree as follows:

                                   ARTICLE 1

                     6.40% Senior Insured Quarterly Notes

          SECTION 101.  Establishment.  There is hereby established a new series
of Securities to be issued under the Indenture, to be designated as the
Company's 6.40% Senior Insured Quarterly Notes due February 1, 2019 (the "Senior
Insured Quarterly Notes").

          There are to be authenticated and delivered $100,000,000 principal
amount of Senior Insured Quarterly Notes to be issued at 100% of principal
amount, and no further Senior Insured Quarterly Notes shall be authenticated and
delivered except as provided by Section 307 of the Original Indenture. The
Senior Insured Quarterly Notes shall be issued in definitive fully registered
form.

          The Senior Insured Quarterly Notes shall be issued in the form of one
Global Security in substantially the form set out in Exhibit A hereto. The
initial Depositary with respect to the Senior Insured Quarterly Notes shall be
The Depository Trust Company.

          The Company will not pay Additional Amounts, as defined in Section
1008 of the Original Indenture.

<PAGE>
 
          The form of the Trustee's Certificate of Authentication for the Senior
Insured Quarterly Notes shall be in substantially the form set forth in 
Exhibit B hereto.

          The Senior Insured Quarterly Notes shall be dated the date of
authentication thereof and shall bear interest from the Original Issue Date.

          The interest rate on the Senior Insured Quarterly Notes will not be
reset pursuant to Section 308(b) of the Original Indenture and the stated
maturity shall not be extended pursuant to Section 309 of the Original
Indenture.

          SECTION 102.  Definitions.  The following defined terms used herein
shall, unless the context otherwise requires, have the meanings specified below.
Capitalized terms used herein for which no definition is provided herein shall
have the meanings set forth in the Original Indenture.

          "Insurance Trustee" means State Street Bank and Trust Company, N.A.,
61 Broadway, 15th Floor, New York, NY 10006, or any successor thereto, as the
insurance paying agent under the Policy.

          "Insurer" means MBIA Insurance Corporation.

          "Interest Payment Dates" means February 1, May 1, August 1 and
November 1 of each year.

          "Original Issue Date" means February 17, 1999.

          "Policy" shall mean the financial guaranty insurance policy issued by
the Insurer insuring the payment when due of the principal of and interest on
the Senior Insured Quarterly Notes as provided therein.

          "Regular Record Date" means the 15th calendar day of the month
preceding the respective Interest Payment Date (whether or not a Business Day).

          "Stated Maturity" means February 1, 2019.

          SECTION 103.  Payment of Principal and Interest.  The principal of the
Senior Insured Quarterly Notes shall be due at Stated Maturity (unless earlier
redeemed). The unpaid principal amount of the Senior Insured Quarterly Notes
shall bear interest at the rate of 6.40% per annum until paid or duly provided
for. Interest shall be paid quarterly in arrears on each Interest Payment Date
to the Person in whose name the Senior Insured Quarterly Notes are registered on
the Regular Record Date for such Interest Payment Date. Any such interest that
is not so punctually paid or duly provided for will forthwith cease to be
payable to the Holders on such Regular Record Date and may be paid to the Person
or Persons in whose name the Senior Insured Quarterly Notes are registered at
the close of business on a Special Record Date for the payment of such defaulted
interest to be fixed by the Trustee, notice whereof shall be given to Holders of
the Senior Insured Quarterly Notes not less than ten days prior to such Special
Record Date.

                                       2
<PAGE>
 
          Payments of interest on the Senior Insured Quarterly Notes will
include interest accrued to but excluding the respective Interest Payment Dates.
Interest payments for the Senior Insured Quarterly Notes shall be computed and
paid on the basis of a 360-day year of twelve 30-day months.

          Payment of the principal and interest due at the Stated Maturity or
earlier redemption of the Senior Insured Quarterly Notes shall be made upon
surrender of the Senior Insured Quarterly Notes at the office or agency of the
Company in the Borough of Manhattan, City and State of New York or the Corporate
Trust Office of the Trustee. The principal of and interest on the Senior Insured
Quarterly Notes shall be paid in such currency of the United States of America
as at the time of payment is legal tender for payment of public and private
debts. Payments of interest will be made at the option of the Company, (i) by
check mailed to the address of the Person entitled thereto as such address shall
appear in the Security Register or (ii) by wire transfer to an account located
in the United States maintained by the payee.

          SECTION 104.  Denominations.  The Senior Insured Quarterly Notes may
be issued in denominations of $1,000, or any integral multiple thereof.

          SECTION 105.  Global Securities.  The Senior Insured Quarterly Notes
will be issued in the form of one or more Global Securities registered in the
name of the Depositary or its nominee. Except under the limited circumstances
described below, Senior Insured Quarterly Notes represented by the Global
Security will not be exchangeable for, and will not otherwise be issuable as,
Senior Insured Quarterly Notes in definitive form. The Global Securities
described above may not be transferred except by the Depositary to a nominee of
the Depositary or by a nominee of the Depositary to the Depositary or another
nominee of the Depositary or to a successor Depositary or its nominee.

          Owners of beneficial interests in such a Global Security will not be
considered the Holders thereof for any purpose under the Indenture, and no
Global Security representing Senior Insured Quarterly Notes shall be
exchangeable, except for another Global Security of like denomination and tenor
to be registered in the name of the Depositary or its nominee or to a successor
Depositary or its nominee. The rights of Holders of such Global Security shall
be exercised only through the Depositary.

          A Global Security shall be exchangeable for Senior Insured Quarterly
Notes registered in the names of persons other than the Depositary or its
nominee only if (i) the Depositary notifies the Company that it is unwilling or
unable to continue as a Depositary for such Global Security and no successor
Depositary shall have been appointed by the Company, or if at any time the
Depositary ceases to be a clearing agency registered under the Securities
Exchange Act of 1934, as amended, at a time when the Depositary is required to
be so registered to act as such Depositary and no successor Depositary shall
have been appointed by the Company, in each case within 90 days after the
Company receives such notice or becomes aware of such cessation, (ii) the
Company in its sole discretion determines that such Global Security shall be so
exchangeable or (iii) there shall have occurred an Event of Default with respect
to the Senior Insured Quarterly Notes.

          SECTION 106.  Payments to the Trustee.  The Company shall make all
payments due on the Senior Insured Quarterly Notes by 2 p.m. New York time on
the second day
 
                                       3
<PAGE>
 
next proceeding any date on which payment of principal of or interest on the
Senior Insured Quarterly Notes is due.

          SECTION 107.  Transfer.  No service charge will be made for any
transfer or exchange of Senior Insured Quarterly Notes, but payment will be
required of a sum sufficient to cover any tax or other governmental changes that
may be imposed in connection therewith.

          SECTION 108.  Redemption at the Company's Option.  The Senior Insured
Quarterly Notes shall be subject to redemption at the option of the Company, in
whole or in part, without premium or penalty, at any time or from time to time
on or after February 1, 2003, at a Redemption Price equal to 100% of the
principal amount to be redeemed plus accrued but unpaid interest to the
Redemption Date.

          In the event of redemption of the Senior Insured Quarterly Notes in
part only, new Senior Insured Quarterly Notes for the unredeemed portion will be
issued in the name or names of the Holders thereof upon the surrender thereof.

          SECTION 109.  Redemption at the Holder's Option.  For purposes of this
Section 109, a "Beneficial Owner" means the Person who has the right to sell,
transfer or otherwise dispose of an interest in Senior Insured Quarterly Notes
and the right to receive the proceeds therefrom, as well as the interest and
principal payable to the Holder thereof. In general, a determination of
beneficial ownership in the Senior Insured Quarterly Notes will be subject to
the rules, regulations and procedures governing the Depositary and institutions
that have accounts with the Depositary or a nominee thereof ("Participants").

          Unless the Senior Insured Quarterly Notes have been declared due and
payable prior to their maturity by reason of an Event of Default, the
Representative (as hereinafter defined) of a deceased Beneficial Owner has the
right to request redemption prior to Stated Maturity of all or part of his
interest, expressed in integral multiples of $1,000 principal amount, in the
Senior Insured Quarterly Notes, and the Company will redeem the same subject to
the limitations that the Company will not be obligated to redeem, during the
period from the Original Issue Date until (but not including) February 1, 2000
(the "Initial Period"), and during each twelve-month period thereafter up to
(but not including) each February 1 thereafter (each such twelve-month period
being hereinafter referred to as a "Subsequent Period"), (i) on behalf of a
deceased Beneficial Owner any interest in the Senior Insured Quarterly Notes
which exceeds an aggregate principal amount of $30,000 or (ii) interests in the
Senior Insured Quarterly Notes in an aggregate principal amount exceeding
$3,000,000. A request for redemption may be initiated by the Representative of a
deceased Beneficial Owner at any time and in any principal amount in integral
multiples of $1,000. If the Company, although not obligated to do so, chooses to
redeem interests of any deceased Beneficial Owner in the Senior Insured
Quarterly Notes in the Initial Period or any Subsequent Period in excess of the
$30,000 limitation, such redemption, to the extent that it exceeds the $30,000
limitation for any deceased Beneficial Owner, shall not be included in the
computation of the $3,000,000 limitation for such Initial Period or such
Subsequent Period, as the case may be, or for any succeeding Subsequent Period.

          Subject to the $30,000 and $3,000,000 limitations, the Company will,
after the death of any Beneficial Owner, redeem the interest of such Beneficial
Owner in the Senior Insured Quarterly Notes within 60 days following receipt by
the Trustee of a Redemption Request (as herein defined). The Trustee will notify
the Company promptly after receipt of any

                                       4
<PAGE>
 
Redemption Request and the Company will provide all funds necessary for such
redemption prior to the date of redemption to the Paying Agent. If Redemption
Requests exceed the aggregate principal amount of interests in Senior Insured
Quarterly Notes required to be redeemed during the Initial Period or during any
Subsequent Period, then such excess Redemption Requests will be applied in the
order received by the Trustee to successive Subsequent Periods, regardless of
the number of Subsequent Periods required to redeem such interests. All
Redemption Requests will be redeemed in the order in which the trustee receives
the Redemption Request.

          A request for redemption of an interest in the Senior Insured
Quarterly Notes may be initiated by the personal representative or other Person
authorized to represent the estate of the deceased Beneficial Owner or from a
surviving joint tenant(s) or tenant(s) by the entirety or tenant(s) in common
(each, a "Representative"). The Representative shall deliver a request to the
Participant through whom the deceased Beneficial Owner owned such interest, in
form satisfactory to the Participant, together with evidence of the death of the
Beneficial owner, evidence of the authority of the Representative satisfactory
to the Participant, such waivers, notices or certificates as may be required
under applicable state or federal law and such other evidence of the right to
such redemption as the Participant shall require. The request shall specify the
principal amount of the interest in the Senior Insured Quarterly Notes to be
redeemed. The Participant shall thereupon deliver to the Depositary a request
for redemption substantially in the form attached as Exhibit C hereto (a
"Redemption Request"), accompanied by the documents submitted to the Participant
as above provided, and the Depositary will forward substantially the same to the
Trustee, in addition to such information as the Trustee and the Company shall
reasonably require. Documents accompanying Redemption Requests shall be in form
satisfactory to the Company. The Trustee may conclusively assume, without
independent investigation, that the statements contained in each Redemption
Request are true and correct and shall have no responsibility for reviewing any
documents accompanying a Redemption Request or for determining whether the
applicable decedent is in fact the Beneficial Owner of the interest in the
Senior Insured Quarterly Notes to be redeemed or is in fact deceased and whether
the Representative is duly authorized to request redemption on behalf of the
applicable Beneficial Owner.

          The price to be paid by the Company for interests in the Senior
Insured Quarterly Notes to be redeemed pursuant to a Redemption Request is 100%
of the principal amount thereof plus accrued but unpaid interest to the date of
payment. Subject to arrangements with the Depositary, payment for interests in
the Senior Insured Quarterly Notes which are to be redeemed shall be made to the
Depositary upon presentation of Senior Insured Quarterly Notes to the Trustee
for redemption in the aggregate principal amount specified in the Redemption
Requests submitted to the Trustee by the Depositary which are to be fulfilled in
connection with such payment. The principal amount of any Senior Insured
Quarterly Notes acquired or redeemed by the Company other than by redemption at
the option of any Representative of a deceased Beneficial Owner pursuant to this
Section 109 shall not be included in the computation of either the $30,000 or
the $3,000,000 limitation for the Initial Period or for any Subsequent Period.

          For purposes of this Section 109, an interest in Senior Insured
Quarterly Notes held in tenancy by the entirety, joint tenancy or by tenants in
common will be deemed to be held by a single Beneficial Owner and the death of a
tenant by the entirety, joint tenant or tenant in common will be deemed the
death of a Beneficial Owner. The death of a Person who, during his

                                       5
<PAGE>
 
lifetime, was entitled to substantially all of the rights of a Beneficial Owner
of an interest in the Senior Insured Quarterly Notes will be deemed the death of
the Beneficial Owner, regardless of the recordation of such interest on the
records of the Participant, if such rights can be established to the
satisfaction of the Participant and the Company. Such interests shall be deemed
to exist in typical cases of nominee ownership, ownership under the Uniform
Gifts to Minors Act or the Uniform Transfers to Minors Act, community property
or other similar joint ownership arrangements, including individual retirement
accounts or Keogh [H.R. 10] plans maintained solely by or for the decedent or by
or for the decedent and any spouse, and trust and certain other arrangements
where one Person has substantially all of the rights of a Beneficial Owner
during such Person's lifetime.

          In the case of any Redemption Request which is presented pursuant to
this Section 109 and which has not been fulfilled at the time the Company gives
notice of its election to redeem Senior Insured Quarterly Notes pursuant to
Section 108 hereof, such interest or portion thereof shall not be subject to
redemption pursuant to such Section 108, but shall remain subject to redemption
pursuant to this Section 109.

          Subject to the provisions of the immediately preceding sentence, any
Redemption Request may be withdrawn by the Person(s) presenting the same upon
delivery of a written request for such withdrawal given by the Depositary to the
Trustee prior to payment of such Redemption Request.

          During such time or times as, in accordance with Section 105 hereof,
the Senior Insured Quarterly Notes are not represented by a Global Security and
are issued in definitive form, all references in this Section 109 to
Participants and the Depositary, including the Depositary's governing rules,
regulations and procedures shall be deemed deleted, all determinations which
under this Section 109 the Participants are required to make shall be made by
the Company (including, without limitation, determining whether the applicable
decedent is in fact the Beneficial Owner of the interest in the Senior Insured
Quarterly Notes to be redeemed or is in fact deceased and whether the
Representative is duly authorized to request redemption on behalf of the
applicable Beneficial Owner), all redemption requests, to be effective, shall be
delivered by the Representative to the Trustee, with a copy to the Company, and
shall be in the form of a Redemption Request (with appropriate changes to
reflect the fact that such Redemption Request is being executed by a
Representative) and, in addition to all documents that are otherwise required to
accompany a Redemption Request, shall be accompanied by the Senior Insured
Quarterly Notes that are the subject of such request.

          SECTION 110.  Other Terms.  The Senior Insured Quarterly Notes will
not have a sinking fund.

          Notice of redemption shall be given as provided in Section 1104 of the
Original Indenture.

          Any redemption of less than all of the Senior Insured Quarterly Notes
shall, with respect to the principal thereof, be divisible by $1,000.

          Except as provided herein, the Senior Insured Quarterly Notes will not
be redeemable.

                                       6
<PAGE>
 
                                  ARTICLE  2

                         Special Insurance Provisions

          SECTION 201.  Insurer as Third Party Beneficiary.  To the extent that
the Indenture confers upon or gives or grants to the Insurer any right, remedy
or claim, the Insurer is hereby explicitly recognized as being a third-party
beneficiary hereunder and may enforce any such right remedy or claim conferred,
given or granted hereunder.

          SECTION 202.  Notices and Information.  (a) The Company shall furnish
to the Insurer:

               (1)  Any notice that is required to be given to a Holder of the
          Senior Insured Quarterly Notes or to the Trustee pursuant to the
          Indenture.

               (2)  As soon as practicable after the filing thereof, a copy of
          any financial statement of the Company and a copy of any audit and
          annual report of the Company; a copy of any notice to be given to the
          registered owners of the Senior Insured Quarterly Notes including,
          without limitation, notice of any redemption of or defeasance of the
          Senior Insured Quarterly Notes; and such additional information it may
          reasonably request.

          (b)  The Company will permit the Insurer to have access to and to make
copies of all books and records relating to the Senior Insured Quarterly Notes
at any reasonable time.

          (c)  The Insurer shall have the right to direct an accounting at the
Company's expense, and the Company's failure to comply with such direction
within thirty days after receipt of written notice of the direction from the
Insurer shall be deemed a default hereunder; provided, however, that if
compliance cannot occur within such period, then such period will be extended so
long as compliance is begun within such period and diligently pursued, but only
if such extension would not materially adversely affect the interests of any
registered owner of the Senior Insured Quarterly Notes.

          (d)  Notwithstanding any other provision of the Indenture, the Trustee
and the Company shall immediately notify the Insurer in accordance with Section
206 if at any time after such amounts are due to be paid to the Trustee or
Paying Agent there are insufficient moneys to make any payments of principal
and/or interest as required and promptly upon the occurrence of any Event of
Default hereunder.

          All notices and information required to be given to the Insurer 
shall be in writing and shall be sent by overnight delivery to MBIA Insurance
Corporation, 113 King Street, Armonk, New York 10504 (Fax number: (914) 
765-3799).

          SECTION 203.  Concerning the Special Insurance Provisions.  The
provisions of this Article 2 shall apply notwithstanding anything in the
Indenture to the contrary, but only so long as the Policy shall be in full force
and effect and the Insurer is not in default thereunder.

          SECTION 204.  Amendments or Supplements.  Any provision of the
Indenture may not be amended in any manner without the prior written consent of
the Insurer.

                                       7
<PAGE>
 
          SECTION 205.  Defeasance.  Notwithstanding anything herein to the
contrary, in the event that the principal and/or interest due on the Senior
Insured Quarterly Notes shall be paid by the Insurer pursuant to the Policy, the
Senior Insured Quarterly Notes shall remain Outstanding for all purposes, not be
defeased or otherwise satisfied and not be considered paid by the Company, and
the assignment and pledge of moneys held in trust by the Trustee and all
covenants, agreements and other obligations of the Company to the registered
owners shall continue to exist and shall run to the benefit of the Insurer, and
the Insurer shall be subrogated to the rights of such registered owners.

          SECTION 206.  Rights of Insurer Controlling.  Anything herein to the
contrary notwithstanding, if the Insurer is not in default of its obligation to
make payments under the Policy, for all purposes except with respect to the
rights provided in Section 902 of the Original Indenture for actions that
require consent of all of the Holders of Outstanding Senior Insured Quarterly
Notes and the right of redemption contained in Section 109 of this Fourth
Supplemental Indenture, the Insurer shall be deemed to be the owner of all
Senior Insured Quarterly Notes then Outstanding for all purposes (including,
without limitation, all approvals, consents, waivers, authorizations,
directions, inspections and the institution of any action), and shall have the
exclusive right to exercise or direct the exercise of remedies on behalf of the
owners of Senior Insured Quarterly Notes in accordance with the terms hereof
following an Event of Default, and the principal of all Senior Insured Quarterly
Notes Outstanding may not be declared to be due and payable immediately without
the prior written consent of the Insurer.

          SECTION 207.  Payments Under the Insurance Policy.  (a) If, as of the
second day next preceding any date on which payment of principal of or interest
on the Senior Insured Quarterly Notes is due, there are insufficient moneys
available hereunder to pay all principal and interest coming due on the Senior
Insured Quarterly Notes on the next succeeding Interest Payment Date or at
Stated Maturity, the Trustee shall immediately notify the Insurer or its
designee by telephone or telegraph, confirmed in writing by registered or
certified mail, of the amount of the deficiency.

          (b)  If the deficiency is made up in whole or in part prior to or on
the Interest Payment Date or at Stated Maturity, the Trustee shall so notify the
Insurer or its designee.

          (c)  In addition, if the Trustee has notice that any of the
Noteholders have been required to disgorge payments of principal or interest on
Senior Insured Quarterly Notes to the Company or to the Trustee in bankruptcy
for creditors or others pursuant to a final judgment by a court of competent
jurisdiction that such payment constitutes a voidable preference to such
Noteholders within the meaning of any applicable bankruptcy laws, then the
Trustee shall notify the Insurer or its designee of such fact by telephone or
telegraphic notice, confirmed in writing by registered or certified mail.

          (d)  The Trustee is hereby irrevocably designated, appointed, directed
and authorized to act as attorney-in-fact for Noteholders of the Senior Insured
Quarterly Notes as follows:

          (i)  if and to the extent there is a deficiency in amounts required to
     pay interest on the Senior Insured Quarterly Notes, the Trustee shall (A)
     execute and deliver to the Insurance Trustee, in form satisfactory to the
     Insurance Trustee, an instrument appointing the Insurer as agent for such
     Noteholders in any legal proceeding related to the payment

                                       8
<PAGE>
 
     of such interest and an assignment to the Insurer of the claims for
     interest to which such deficiency relates and which are paid by the
     Insurer, (B) receive as designee of the respective Noteholders (and not as
     Trustee) in accordance with the tenor of the Policy payment from the
     Insurance Trustee with respect to the claims for interest so assigned and
     (C) disburse the same to such respective Noteholders; and

          (ii)  if and to the extent of a deficiency in amounts required to pay
     principal of the Senior Insured Quarterly Notes, the Trustee shall (A)
     execute and deliver to the Insurance Trustee an instrument appointing the
     Insurer as agent for the Noteholders in any legal proceeding relating to
     the payment of such principal and an assignment to the Insurer of any of
     the Senior Insured Quarterly Notes surrendered to the Insurance Trustee of
     so much of the principal amount thereof as has not previously been paid or
     for which moneys are not held by the Trustee and available for such payment
     (but such assignment shall be delivered only if payment from the Insurance
     Trustee is received for such amount, (B) receive as designee of the
     respective Noteholders (and not as Trustee) in accordance with the tenor of
     the Policy payment therefor from the Insurance Trustee and (C) disburse the
     same to such Noteholders.

          (e)  Payments with respect to claims for interest on and principal of
Senior Insured Quarterly Notes disbursed by the Trustee from proceeds of the
Policy shall not be considered to discharge the obligation of the Company with
respect to such Senior Insured Quarterly Notes, and the Insurer shall become the
owner of such unpaid Senior Insured Quarterly Notes and claims for interest to
the extent of its payment thereof in accordance with the tenor of the assignment
made to it under the provisions of this subsection or otherwise.

          (f)  Irrespective of whether any such assignment is executed and
delivered, the Company and the Trustee hereby agree for the benefit of the
Insurer that:

          (i)  they recognize that to the extent the Insurer makes payments,
     directly or indirectly (as by paying through the Trustee), on account of
     principal of or interest on the Senior Insured Quarterly Notes, the Insurer
     will be subrogated to the rights of such Noteholders to receive the amount
     of such principal and interest from the Company, with interest thereon as
     provided and solely from the sources stated herein and in the Senior
     Insured Quarterly Notes; and

          (ii)  they recognize that to the extent the Insurer makes payment the
     Company will be obligated accordingly to pay to the Insurer the amount of
     such principal and interest (including principal and interest recovered
     under subparagraph (ii) of the first paragraph of the Insurance Policy,
     which principal and interest shall be deemed past due and not to have been
     paid), with interest thereon as provided herein and in the Senior Insured
     Quarterly Notes, but only from the sources and in the manner provided
     herein for the payment of principal of and interest on the Senior Insured
     Quarterly Notes to Noteholders and will otherwise treat the Insurer as the
     owner of such rights to the amount of such principal and interest.

          SECTION 208.  Insurer's Rights Concerning the Trustee.
                        --------------------------------------- 

          (a)  The Trustee or Paying Agent may be removed at any time, at the
request of the Insurer, for any breach of its duties as set forth in the
Indenture.

                                       9
<PAGE>
 
          (b)  The Insurer shall receive prompt written notice of any Trustee or
Paying Agent resignation.

          (c)  Every successor Trustee appointed pursuant to the Section shall
be a trust company or bank in good standing located in or incorporated under the
laws of the United States or any State thereof, duly authorized to exercise
trust powers and subject to examination by federal or state authority, having a
reported capital and surplus of not less than $75,000,000 and acceptable to the
Insurer. Any successor Paying Agent shall not be appointed unless the Insurer
approves such successor in writing.

          (d)  Notwithstanding any other provision of the Indenture, in
determining whether the rights of the Holders of Senior Insured Quarterly Notes
will be adversely affected by any action taken pursuant to the terms and
provisions of the Indenture, the Trustee or Paying Agent shall consider the
effect on the Holders of Senior Insured Quarterly Notes as if there were no
Policy.
         
          (e)  Notwithstanding any other provision of the Indenture, no removal,
resignation or termination of the Trustee or Paying Agent shall take effect
until a successor, acceptable to the Insurer, shall be appointed.

          SECTION 209.  Insurer's Right to Accelerate, etc.  Anything in the
Indenture to the contrary notwithstanding, upon the occurrence and continuance
of an Event of Default, so long as the Policy shall be in full force and effect
and the Insurer is not in default under the terms of the Policy, the Insurer
shall be entitled to control and direct the enforcement of all rights and
remedies granted to the Holders of Senior Insured Quarterly Notes or the Trustee
for the benefit of the Holders of Senior Insured Quarterly Notes under the
Indenture, including, without limitation: (i) the right to accelerate the
principal of the Senior Insured Quarterly Notes as described in the Indenture,
and (ii) the right to annul any declaration of acceleration, and the Insurer
shall also be entitled to approve all waivers of Events of Default.

                                   ARTICLE 3

                           Miscellaneous Provisions

          SECTION 301.  Recitals by Company.  The recitals in this Fourth
Supplemental Indenture are made by the Company only and not by the Trustee, and
all of the provisions contained in the Original Indenture in respect of the
rights, privileges, immunities, powers and duties of the Trustee shall be
applicable in respect of Senior Insured Quarterly Notes and of this Fourth
Supplemental Indenture as fully and with like effect as if set forth herein in
full.

          SECTION 302.  Ratification and Incorporation of Original Indenture.
The Original Indenture is in all respects ratified and confirmed, and the
Original Indenture and this Fourth Supplemental Indenture shall be read, taken
and construed as one and the same instrument; provided that, in case of conflict
between this Fourth Supplemental Indenture and the Original Indenture, this
Fourth Supplemental Indenture shall control.

          SECTION 303.  Executed in Counterparts.  This Fourth Supplemental
Indenture may be simultaneously executed in several counterparts, each of which
shall be

                                      10
<PAGE>
 
deemed to be an original, and such counterparts shall together constitute one
and the same instrument.

          SECTION 304.  Parties Interested Herein.  Nothing in the Indenture
expressed or implied is intended or shall be construed to confer upon, or to
give or grant to, any person or entity, other than the Company, the Trustee, the
Insurer, the Paying Agent and the registered owners of the Senior Insured
Quarterly Notes, any right, remedy or claim under or by reason of the Indenture
or any covenant, condition or stipulation hereof, and all covenants,
stipulations, promises and agreements in the Indenture contained by and on
behalf of the Company shall be for the sole and exclusive benefit of the
Company, the Trustee, the Insurer, the Paying Agent and the registered owners of
the Senior Insured Quarterly Notes.

                                      11
<PAGE>
 
          IN WITNESS WHEREOF, each party hereto has caused this Fourth
Supplemental Indenture to be signed in its name and behalf by its duly
authorized officers or signatories, all as of the day and year first above
written.

                                       ONEOK, INC.


                                       By:
                                           ------------------------------------
                                       Name:   Jerry D. Neal
                                       Title:  Vice President and Treasurer


                                       CHASE BANK OF TEXAS, NATIONAL
                                       ASSOCIATION, as Trustee


                                       By:
                                           ------------------------------------
                                       Authorized Signatory

                                      12

<PAGE>
 
                                                                     Exhibit 5.1
                        [Letterhead of Gable & Gotwals
                          A Professional Corporation]



                                April 15, 1999



ONEOK, Inc.
100 West Fifth Street
Tulsa, OK 74103-4298

     Re:  Form S-3, Registration Statement Under the Securities
          Act of 1933, relating to certain Debt Securities

Gentlemen:

     We are retained as regular counsel for ONEOK, Inc., an Oklahoma corporation
(hereinafter called the "Company") which has filed with the Securities and
Exchange Commission under the Securities Act of 1933, as amended, a Registration
Statement on Form S-3 (including a Prospectus) relating to the registration of
unsecured debentures, notes, bonds or other evidence of indebtedness (the
"Securities") which may be offered as one or more separate series or as a single
series, as determined at the time of the offering.

     We have examined (a) the above-mentioned Registration Statement which is
being filed with the Securities and Exchange Commission; (b) the Indenture (the
"Indenture") entered into between the Company and the Trustee; (c) the
Certificate of Incorporation, as amended, and the By-laws, as amended, of the
Company; (d) the corporate actions taken by the Board of Directors of the
Company in connection with the issuance and sale of the Securities; and (e) such
other corporation records, certificates of public officials and officers of the
Company, and other documents as we have considered relevant to the matters
covered by this opinion.

     In connection with the foregoing, we wish to advise you as follows:

     1.   The Company is a corporation validly organized and existing under the
laws of the State of Oklahoma and is duly qualified to do business as a foreign
corporation in the State of Kansas.

     2.   The filing of the above-mentioned Registration Statement and the
execution of the above-mentioned Indenture have been duly authorized by the
proper corporate action on the part of the Company.

     3.   When a series of Securities has been duly issued pursuant to the terms
and conditions of the Indenture, such Securities shall be legally issued, fully
paid, and non-assessable obligations of the Company in the hands of the then
owners thereof and such Securities shall be valid, legal and binding obligations
of the Company enforceable in accordance with their terms, except as such
<PAGE>
 
ONEOK, Inc.
April 15, 1999
Page 2


enforcement may be limited by bankruptcy, insolvency, reorganization or other
laws of general application relating to or affecting creditors' rights and to
general equity principles.

     We hereby consent to:

     1.   Being named in the above Form S-3 Registration Statement and the
Prospectus which is being made a part thereof, and in any amendments thereto,
under the caption "Legal Matters," as counsel for the Company, passing upon
legal matters in connection with the Securities and having reviewed the matters
of law and legal conclusions under "Description of Securities" contained in said
Prospectus which are included therein under our authority as experts.

     2.   The filing of this opinion as an exhibit to the above-mentioned Form 
S-3 Registration Statement.

                                    Very truly yours,



                                    Donald A. Kihle
                                    For the Firm

DAK:jh

<PAGE>
 
                                                                    EXHIBIT 23.1


                         Independent Auditors' Consent


The Board of Directors
ONEOK, Inc.


We consent to the incorporation by reference herein of our report on the
Consolidated Financial Statements of ONEOK, Inc. and subsidiaries as of August
31, 1998 and 1997, and for each of the years in the three-year period ended
August 31, 1998, which report appears in the August 31, 1998, Annual Report on
Form 10-K of ONEOK, Inc. and to the reference to our firm under the heading
"Experts" in the prospectus.


                                                                  /s/ KPMG LLP

Tulsa, Oklahoma
April 15, 1999

<PAGE>
 
                                                                    EXHIBIT 23.2


                   CONSENT OF INDEPENDENT PUBLIC ACCOUNTANTS


As independent public accountants, we hereby consent to the incorporation by
reference in this registration statement of our report dated March 26, 1999,
incorporated by reference in Southwest Gas Corporation's Annual Report on Form
10-K for the year ended December 31, 1998 and ONEOK, Inc.'s Current Report on
Form 8-K dated April 15, 1999, and to all references to our firm included in
this registration statement.

                                                     /s/ Arthur Andersen LLP

Las Vegas, Nevada
April 15, 1999

<PAGE>
 
                                                                    Exhibit 25.1

                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                           -------------------------

                                 F O R M   T-1

STATEMENT OF ELIGIBILITY AND QUALIFICATION UNDER THE TRUST INDENTURE ACT OF 1939
                               OF A CORPORATION
                         DESIGNATED TO ACT AS TRUSTEE

                           -------------------------

   CHECK IF AN APPLICATION TO DETERMINE ELIGIBILITY OF A TRUSTEE PURSUANT TO
                            SECTION 305(B)(2)____.
                                        
                   CHASE BANK OF TEXAS, NATIONAL ASSOCIATION
              (Exact name of trustee as specified in its charter)

Organized under the laws of                           74-0800980
the United States of America                        (I.R.S. employer
(State of incorporation                            identification no.)
if not a National Bank)

712 Main Street                                          77002
HOUSTON, TEXAS                                        (Zip Code)
(Address of principal executive offices)

Lee Boocker
Chase Bank of Texas, National Association
712 Main Street, 26th Floor
Houston, Texas 77002
(713) 216-2448
(Name, address and telephone
number of agent for service)

                           -------------------------

                                  ONEOK, INC.
              (Exact name of obligor as specified in its charter)



Oklahoma                                              73-1520922

(State or other jurisdictions of                    (I.R.S. employer
incorporation or organization)                    identification nos.)
 
100 West Fifth Street
Tulsa, Oklahoma                                          74103
(Address of obligor's principal executive offices)     (Zip Code)


                         $500,000,000 DEBT SECURITIES
                      (Title of the indenture securities)

                                       1
<PAGE>
 
Item 1.    General Information.

           Furnish the following information as to the Trustee:

           (a)  Name and address of each examining or supervising authority to
                which it is subject.

           Name                                                   Address
           -------------------------------------------------------------------
           Comptroller of the Currency                        Washington, D.C.
           Board of Governors of the Federal Reserve System   Washington, D.C.
           Federal Deposit Insurance Corporation              Washington, D.C.
 
           (b)  Whether it is authorized to exercise corporate trust powers.

           Yes, the trustee is authorized to exercise corporate trust powers.


Item 2.    Affiliations with the Obligor.

           If the obligor is an affiliate of the Trustee, describe each such
           affiliation. (See Note on Page 3)

           The obligor is not an affiliate of the trustee.


Item 16.   List of Exhibits.

           List below all exhibits filed as part of this statement of
           eligibility:

           Exhibit 1.  A copy of the Articles of Association of the Trustee as
                       now in effect.
           Exhibit 2.  A copy of the certificate of authority of the Trustee to
                       commence business.
           Exhibit 3.  A copy of the authorization of the Trustee to exercise
                       corporate trust powers.
           Exhibit 4.  A copy of the existing bylaws of the Trustee.
           Exhibit 5.  Not Applicable.
           Exhibit 6.  The consents of the United States Institutional trustees
                       required by Section 321(b) of the Trust Indenture Act of
                       1939.
           Exhibit 7.  A copy of the latest report of condition of the Trustee
                       published pursuant to law or the requirements of its
                       supervising or examining authority.
           Exhibit 8.  Not Applicable.
           Exhibit 9.  Not Applicable.

                                       2
<PAGE>
 
                      NOTE REGARDING INCORPORATED EXHIBIT

Effective January 20, 1998, the name of the Trustee was changed from Texas
Commerce Bank National Association to Chase Bank of Texas, National Association.
The exhibits incorporated herein by reference, except for Exhibit 7, were filed
under the former name of the Trustee.

     Exhibit 1.  Incorporated by reference to exhibit bearing the same
                 designation and previously filed with the 
                 Securities and Exchange Commission as exhibit
                 to the Form S-3 File No. 33-56195.   
     Exhibit 2.  Incorporated by reference to exhibit bearing the same
                 designation and previously filed with the 
                 Securities and Exchange Commission as exhibit
                 to the Form S-3 File No. 33-42814.   
     Exhibit 3.  Incorporated by reference to exhibit bearing the same
                 designation and previously filed with the 
                 Securities and Exchange Commission as exhibit
                 to the Form S-11 File No. 33-25132.   
     Exhibit 4.  Incorporated by reference to exhibit bearing the same
                 designation and previously filed with the 
                 Securities and Exchange Commission as exhibit
                 to the Form S-3 File No. 33-65055.   
     Exhibit 6.  Incorporated herewith.
     Exhibit 7.  Incorporated by reference to exhibit bearing the same
                 designation and previously filed with the 
                 Securities and Exchange Commission as exhibit
                 to the Form S-3 File No. 333-63747.  

Note:  The answer to Item 2 is based in part on information provided or
       confirmed by the obligor. The accuracy and completeness of such
       information is hereby disclaimed by the Trustee.

                                       3
<PAGE>
 
                                 SIGNATURE


  Pursuant to the requirements of the Trust Indenture Act of 1939, the Trustee,
Chase Bank of Texas, National Association, a national banking association
organized and existing under the laws of the United States of America, has duly
caused this statement of eligibility to be signed on its behalf by the
undersigned, thereunto duly authorized, all in the City of Dallas, and State of
Texas, on the 15th day of April 1999.



                                 CHASE BANK OF TEXAS, NATIONAL ASSOCIATION



                                 By: /s/ JOHN G. JONES       
                                    -------------------------------------------
                                 Name:   John G. Jones      
                                 Title:  Vice President


                                       4
<PAGE>
 
                                 EXHIBIT 6


Securities and Exchange Commission
Washington, D.C. 20549

Gentlemen:

  The undersigned is trustee under an indenture between ONEOK, Inc., an Oklahoma
corporation (the "Company") and Chase Bank of Texas, National Association
(formerly known as Texas Commerce Bank National Association), as Trustee,
entered into in connection with the issuance of the Corporation's Debt
Securities.

  In accordance with Section 321(b) of the Trust Indenture Act of 1939, the
undersigned hereby consents that reports of examinations of the undersigned,
made by Federal or State authorities authorized to make such examinations, may
be furnished by such authorities to the Securities and Exchange Commission upon
its request therefor.

                                  CHASE BANK OF TEXAS, NATIONAL ASSOCIATION


                                  By: /s/ JOHN G. JONES       
                                     --------------------------------------
                                  Name:   John G. Jones      
                                  Title:  Vice President
                                  Date:   April 15, 1999

                                       5


© 2022 IncJournal is not affiliated with or endorsed by the U.S. Securities and Exchange Commission