OSI PHARMACEUTICALS INC
S-3, 2000-03-02
IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES
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<PAGE>   1
      As filed with the Securities and Exchange Commission on March 2, 2000
                                                           Registration No. 333-

                       SECURITIES AND EXCHANGE COMMISSION
                               ------------------

                                    FORM S-3

             REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933
                               ------------------


                            OSI PHARMACEUTICALS, INC.
             (Exact name of registrant as specified in its charter)


<TABLE>
<S>                                                                                <C>
                                        DELAWARE                                                13-3159796
               (State or other jurisdiction of incorporation organization)         (I.R.S. Employer Identification No.)
</TABLE>


           106 CHARLES LINDBERGH BOULEVARD, UNIONDALE, NEW YORK, 11553
                                 (516) 222-0023
         (Address, including zip code, and telephone number, including
                 area code, of registrant's principal executive
                                    offices)
                             -----------------------


                             ROBERT L. VAN NOSTRAND
                   VICE PRESIDENT AND CHIEF FINANCIAL OFFICER
                            OSI PHARMACEUTICALS, INC.
           106 CHARLES LINDBERGH BOULEVARD, UNIONDALE, NEW YORK 11553,
                                 (516) 222-0023
           (Name, address, including zip code, and telephone number,
                   including area code, of agent for service)
                             -----------------------

                        Copies of all communications to:

                         SPENCER W. FRANCK, JR., ESQUIRE
                         SAUL, EWING, REMICK & SAUL LLP
     1500 MARKET STREET, 38TH FLOOR, PHILADELPHIA, PENNSYLVANIA 19102-2186,
                                 (215) 972-1955

APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC: From time to
time after the effective date of this registration statement.

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>
  TITLE OF SECURITIES TO BE REGISTERED       AMOUNT TO BE        PROPOSED MAXIMUM          PROPOSED MAXIMUM            AMOUNT OF
                                              REGISTERED     OFFERING PRICE PER SHARE  AGGREGATE OFFERING PRICE    REGISTRATION FEE
- -----------------------------------------------------------------------------------------------------------------------------------

<S>                                          <C>             <C>                       <C>                         <C>
Common Stock, par value $.01 per share         3,325,000            $22.53(1)              $74,916,406.00(1)         $19,777.93(2)
- -----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

(1)  In accordance with Rule 457(c), the price shown is estimated solely for the
     purposes of calculating the registration fee, and is based on the average
     of the reported high and low sales prices of the common stock as reported
     on the Nasdaq National Market on February 24, 2000, which was $22.53.

(2)  Represents the Proposed Maximum Aggregate Offering Price multiplied by
     $0.000264.

         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 THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(a),
MAY DETERMINE.
<PAGE>   2
PROSPECTUS

                                3,325,000 SHARES

                            OSI PHARMACEUTICALS, INC.

                     COMMON STOCK, PAR VALUE $.01 PER SHARE


         The stockholders named on pages 9-10 are selling up to 3,325,000 shares
of OSI's stock.

         OSI's common stock is traded on the Nasdaq National Market under the
symbol "OSIP". On February 28, 2000, the reported closing price of the common
stock was $24.125 per share. OSI's principal executive offices are located at
106 Charles Lindbergh Boulevard, Uniondale, New York 11553, and its telephone
number is (516) 222-0023.

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

                       THIS INVESTMENT INVOLVES RISK.
           SEE "RISK FACTORS" BEGINNING ON PAGE 1 OF THIS PROSPECTUS.
                           --------------------------

         NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.


                  THE DATE OF THIS PROSPECTUS IS MARCH 2, 2000
<PAGE>   3
                               TABLE OF CONTENTS


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<S>                                                                                                                   <C>
RISK FACTORS.....................................................................................................       1


RECENT EVENTS....................................................................................................       9


USE OF PROCEEDS..................................................................................................       9


SELLING STOCKHOLDERS.............................................................................................       9


PLAN OF DISTRIBUTION.............................................................................................      12


AVAILABLE INFORMATION............................................................................................      13


INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE..................................................................      14


LEGAL MATTERS....................................................................................................      15


EXPERTS..........................................................................................................      15
</TABLE>


                                      -i-
<PAGE>   4
                                  RISK FACTORS

         We desire to take advantage of the "Safe Harbor" provisions of the
Private Securities Litigation Reform Act of 1995 and of Section 21E and Rule
3b-6 under the Securities Exchange Act of 1934, as amended. Specifically, we
wish to alert readers that the following important factors, as well as other
factors including, without limitation, those described elsewhere in reports we
have filed with the SEC, could in the future affect, and in the past have
affected, our actual results and could cause our results for future periods to
differ materially from those expressed in any forward-looking statements made by
or on behalf of OSI. We assume no obligation to update these forward-looking
statements.

         ALTHOUGH WE HAVE POTENTIAL PRODUCTS THAT APPEAR TO BE PROMISING AT
EARLY STAGES OF DEVELOPMENT, ALL OF OUR PRODUCTS WILL REQUIRE SIGNIFICANT
RESEARCH AND DEVELOPMENT AND MAY NOT REACH THE MARKET FOR A NUMBER OF REASONS.

         Potential products may be found ineffective or cause harmful side
effects during pre-clinical testing or clinical trials, fail to receive
necessary regulatory approvals, be difficult to manufacture on a large scale, be
uneconomical to produce, fail to achieve market acceptance or be precluded from
commercialization by proprietary rights of third parties. There is no guarantee
that we or our collaborative partners' product development efforts will be
successfully completed, that required regulatory approvals will be obtained or
that any products, if introduced, will be successfully marketed or achieve
customer acceptance.

         To date, we have generated no revenue from the sale of pharmaceutical
products, except for CP-358,774, with respect to which Pfizer has completed
Phase I safety and toxicity studies and has initiated Phase II clinical trials,
and CP-609,754 for which Pfizer has opened an investigational new drug
application for Phase I clinical trials, all of the lead compounds in our small
molecule drug discovery programs are in either a discovery or pre-clinical
evaluation phase. Any products resulting from our development programs are not
expected to be commercially available for several years, if at all.

         Our live-cell assays are novel as a drug discovery method and have not
yet been shown to be successful in the development of any commercialized drug.
Furthermore, our drug discovery assays are focused on several target genes and
other molecular targets, the functions of many of which have not yet been fully
determined. Our live-cell assay technology may not result in lead compounds that
will be safe and useful. Development of new pharmaceutical products is highly
uncertain. Consequently, our drug discovery technology may not result in any
commercially successful products.

         FAILURE TO OBTAIN REQUIRED GOVERNMENTAL APPROVALS WILL DELAY OR
PRECLUDE OUR PARTNERS FROM MARKETING DRUGS DISCOVERED OR DEVELOPED BY US OR
LIMIT THE COMMERCIAL USE OF THESE PRODUCTS.

         Prior to marketing by a collaborative partner, any new drug discovered
by us must undergo an extensive regulatory approval process in the United States
and other countries. This regulatory process, which includes pre-clinical
testing and clinical trials of each compound to establish its safety and
efficacy, can take many years and require the expenditure of substantial
resources. Moreover, data obtained from pre-clinical and clinical activities are
susceptible to varying interpretations that could delay, limit or prevent
regulatory approval.

         Even if we obtain regulatory approval, a marketed product and its
manufacturer are subject to continuing review, including post-marketing
surveillance. Discovery of previously unknown problems
<PAGE>   5
with our product or its manufacturer may have a negative effect on our business,
financial condition and results of operations, including the withdrawal of the
product from the market. Violations of regulatory requirements at any stage may
result in various unfavorable consequences to us, including the Food and Drug
Administration's, or FDA's, delay in approving or its refusal to approve a
product, withdrawal of an approved product from the market and the imposition of
criminal penalties against the manufacturer and the new drug application holder.
Although Pfizer submitted an investigational new drug application to the FDA
with respect to the epidermal growth factor receptor inhibitor CP-358,774 and
CP-609,754 for farnesylation, we have not submitted an investigational new drug
application for any product candidate, and no product candidate has been
approved for commercialization in the United States or elsewhere. We intend to
file investigational new drug applications for product candidates in our
internal proprietary programs, but to rely on our partners to file
investigational new drug applications in our collaborative programs. No
assurance can be given that we or any of our collaborative partners will be able
to conduct clinical testing or obtain the necessary approvals from the FDA or
other regulatory authorities for any products.

         IF WE CONTINUE TO INCUR SUBSTANTIAL LOSSES AS WE HAVE EXPERIENCED SINCE
INCEPTION, WE MAY NEVER ACHIEVE PRODUCT REVENUES OR PROFITABILITY WHICH MAY
CAUSE THE VALUE OF OUR COMMON STOCK TO DECREASE.

         We have had net operating losses since our inception in 1983. At
September 30, 1999, our accumulated deficit was approximately $65.6 million. The
net income recorded for fiscal quarter ended December 31, 1999 is not indicative
of our expected future results. They are a result of two non-recurring events
including a one-time technology access fee from a collaborative partner and a
gain from the sale of our diagnostics business. Our losses have resulted
principally from costs incurred in research and development, and from general
and administrative costs associated with our operations. These costs have
exceeded our revenues, which to date have been generated principally from
collaborative research agreements.

         We expect to incur substantial additional operating expenses over the
next several years as a result of increases in our expenses for research and
development, including enhancements in our drug discovery technologies and with
respect to our internal proprietary projects. If we do not obtain additional
third party funding for these expenses, we expect that the expenses will result
in increased losses from operations. We do not expect to generate revenues from
the sale of our small molecule products for several years.

         IF WE OR OUR COLLABORATIVE PARTNERS DO NOT SUCCESSFULLY DEVELOP,
COMMERCIALIZE, MANUFACTURE AND MARKET PRODUCT CANDIDATES, WE MAY NEVER ACHIEVE
PRODUCT REVENUES OR PROFITABILITY.

         Our future profitability depends, in part, on:

         -        collaborative partners obtaining regulatory approval for
                  products derived from our collaborative research efforts;

         -        collaborative partners successfully producing and marketing
                  products derived from technology or rights licensed from us;
                  and

         -        entering into agreements for the development,
                  commercialization, manufacture and marketing of any products
                  derived from our internal proprietary programs.

         Our future capital requirements will depend on many factors, which
include:


                                      -2-
<PAGE>   6
         -        continued scientific progress in our research and development
                  programs;

         -        size and complexity of our research and development programs;

         -        progress of pre-clinical testing and early stage clinical
                  trials;

         -        time and costs involved in obtaining regulatory approvals for
                  our product candidates;

         -        costs involved in filing, prosecuting, defending and enforcing
                  patent claims and other intellectual property rights;

         -        competing technological and market developments;

         -        establishment of additional collaborative arrangements;

         -        costs of manufacturing arrangements;

         -        costs of commercialization activities; and

         -        costs of product in-licensing and strategic acquisitions, if
                  any.

         We intend to seek additional funding through arrangements with
corporate collaborators and may seek additional funding through public or
private sales of our securities, including equity securities. Additional funding
may not be available on reasonable or acceptable terms, if at all. Furthermore,
any additional equity financings would be dilutive to our stockholders. If
adequate funds are not available, we may be required to curtail significantly
one or more of our research and development programs or obtain funds through
arrangements with collaborative partners or others that may require us to
relinquish our rights to a number of our technologies or product candidates. If
funding from one or more of our collaborative programs were reduced or
terminated, we would be forced to devote additional internal resources to
product development, scale back or terminate selected development programs or
seek alternative collaborative partners.

         OUR PRODUCTS MAY BE SUBJECT TO DELAYS IN MANUFACTURE IF COLLABORATIVE
PARTNERS OR OUTSIDE CONTRACTORS GIVE OTHER PRODUCTS GREATER PRIORITY THAN OUR
PRODUCTS.

         The manufacture of our candidate products for clinical trials and the
manufacture of resulting products for commercialization purposes are subject to
current good manufacturing practices regulations promulgated by the FDA. We rely
on collaborative partners or outside contractors to manufacture our products in
their FDA-approved manufacturing facilities. Our collaborative agreements allow
our collaborative partners significant discretion in electing to pursue or not
to pursue the activities upon which it relies. We cannot control the amount and
timing of resources our collaborative partners devote to our programs or
potential products. Our products may be in competition with other products for
priority of access to these facilities. For this and other reasons, there can be
no assurance that our collaborative partners will manufacture our products in an
effective or timely manner. If not performed in a timely manner, the clinical
trial development of our product candidates or their submission for regulatory
approval could be delayed, and our ability to deliver products on a timely basis
could be impaired or precluded. We may not be able to enter into any necessary
third-party manufacturing arrangements on acceptable terms if at all. Our
current dependence upon others for the manufacture of our products could
negatively affect our future profit margin, if any, and our ability to
commercialize products on a timely and competitive basis.

         OUR LIMITED EXPERIENCE IN CONDUCTING CLINICAL TRIALS AND RELIANCE ON
THE PHARMACEUTICAL COMPANIES WITH WHICH WE COLLABORATE FOR CLINICAL DEVELOPMENT
AND REGULATORY APPROVALS MAY CAUSE DELAYS, TERMINATIONS OR SETBACKS IN OUR
BUSINESS.


                                      -3-
<PAGE>   7
         To date, only two product candidates have entered clinical trials in
our small molecule drug discovery operations. Only one of the compounds
discovered using our small molecule discovery technology has been proven safe in
humans and none have yet demonstrated efficacy. Our failure to increase the
number of product candidates eligible for clinical trials and unsuccessful
completion of clinical trials in the future could negatively affect our
business, condition and results of operation.

         IF WE ARE UNABLE TO PROTECT OUR INTELLECTUAL PROPERTY RIGHTS, OUR
ABILITY TO DEVELOP AND COMMERCIALIZE OUR TECHNOLOGY AND PRODUCTS WILL BE
SEVERELY LIMITED.

         Our success depends, in part, on our ability or our collaborative
partners' ability to:

         -        obtain patent protection for product candidates;

         -        maintain trade secret protection; and

         -        operate without infringing on the proprietary rights of third
                  parties.

         The degree of future protection for our proprietary rights will remain
uncertain if:

         -        our pending patent applications are not approved for any
                  reason;

         -        we are unable to develop additional proprietary technologies
                  that are patentable; or

         -        patents issued do not provide us with a competitive advantage.

         Furthermore, third parties may:

         -        independently develop similar or alternative technologies;

         -        duplicate some or all of our technologies;

         -        design around our patented technologies if patents are issued
                  to us; and

         -        challenge issued patents.

         WE MAY INCUR SUBSTANTIAL COSTS PROTECTING OUR PROPRIETARY RIGHTS OR
DEFENDING AGAINST CHARGES OF INFRINGEMENT OF OTHER'S PROPRIETARY RIGHTS.

         We are seeking to license to other companies rights to practice under
our gene transcription patent estate. We believe technology and practices
covered by these patents are in widespread use in the pharmaceutical and
biotechnology industries. To date, we have granted four licenses to use our gene
transcription patent. If other pharmaceutical and biotechnology firms which use
our patented technology are not willing to negotiate license arrangements with
us on reasonable terms, we may have to choose between (i) abandoning our
licensing strategy or (ii) initiating legal proceedings against those firms.
Legal action, including patent infringement litigation, is extremely costly.
Consequently, our strategy to commercialize our gene transcription patent estate
through licensing may not be successful.

         IF WE ARE UNABLE TO MAINTAIN OR ENTER INTO ARRANGEMENTS WITH
COLLABORATIVE PARTNERS, OUR ABILITY TO PROCEED WITH RESEARCH AND DEVELOPMENT
PROGRAMS, MANUFACTURING AND THE SALE OF OUR PRODUCT CANDIDATES WILL BE SEVERELY
LIMITED.

         Our business strategy and limited resources require us to enter into
collaborative arrangements with various research partners. We are largely
dependent on our collaborative partners for:

         - pre-clinical testing;


                                      -4-
<PAGE>   8
         - clinical development;

         - regulatory approval;

         - manufacturing and marketing products;

         - compound libraries;

         - patent protection and proprietary technology; and

         - funding.

         Like many small biopharmaceutical companies, our business strategy
includes funding from larger pharmaceutical companies that we collaborate with
to support our research and development programs and the commercialization of
our product candidates. In trying to attract partners to collaborate with, we
face serious competition from other small biopharmaceutical companies as well as
in-house research and development staffs of larger pharmaceutical companies.
Failure to enter into collaborative agreements on acceptable terms could
negatively affect our business, financial condition and results of operation.

         IF ANY OF OUR COLLABORATIVE PARTNERS BREACH OR TERMINATE THEIR
AGREEMENTS WITH US OR OTHERWISE FAIL TO CONDUCT OUR COLLABORATIVE ACTIVITIES
SUCCESSFULLY IN A TIMELY MANNER, OUR PRE-CLINICAL OR CLINICAL DEVELOPMENT OF
PRODUCT CANDIDATES, COMMERCIALIZATION OF PRODUCT CANDIDATES, OR RESEARCH AND
DEVELOPMENT PROGRAMS WOULD BE DELAYED OR TERMINATED.

         We face potential problems with our collaborative partners which could
affect our success including:

         -        competition with our collaborators;

         -        potential disputes with collaborators concerning ownership
                  rights to developed technology;

         -        short term of collaborative agreements which may require their
                  renewal;

         -        delays; and

         -        consolidations of pharmaceutical companies.

         Our success depends, in large part, on the efforts of our collaborative
partners. Potential disagreements between us and collaborators, such as disputes
over ownership rights to any technology developed together, could lead to
litigation and costly delays in the collaborative research and development
programs and the commercialization of product candidates.

         BECAUSE WE GENERALLY AGREE NOT TO CONDUCT INDEPENDENTLY, OR WITH ANY
THIRD PARTY, ANY RESEARCH THAT IS COMPETITIVE WITH THE RESEARCH CONDUCTED UNDER
OUR COLLABORATIVE PROGRAMS, OUR COLLABORATIVE RELATIONSHIPS MAY HAVE THE EFFECT
OF LIMITING THE AREAS OF RESEARCH WE MAY PURSUE.

         Under our collaborative research agreements with most of our partners,
we are prohibited, during the terms of the agreements, from pursuing or
sponsoring research aimed at the discovery of drugs which are the subject of the
collaborations. Our collaborative partners, however, may develop, either alone
or with others, products that are similar to or competitive with the products or
potential products that are the subject of our collaborations with such
partners. Any such competition may lead to the withdrawal by our collaborative
partners of support for our product candidates, which would likely impair our
business, financial condition and results of operations.


                                      -5-
<PAGE>   9
         BECAUSE ALL OF OUR COLLABORATIVE PROGRAMS WITH PHARMACEUTICAL COMPANIES
HAVE TERMS OF SIX OR FEWER YEARS, WHICH IS GENERALLY LESS THAN THE PERIOD
REQUIRED FOR THE DISCOVERY, CLINICAL DEVELOPMENT AND COMMERCIALIZATION OF MOST
DRUGS, THE CONTINUATION OF OUR DRUG DISCOVERY AND DEVELOPMENT PROGRAMS IS
DEPENDENT ON THE PERIODIC RENEWAL OF OUR COLLABORATIVE ARRANGEMENTS.

         All of our collaborative research agreements may be terminated under
various circumstances. Some of our collaborative research agreements provide
that, upon the expiration of a specified period after execution of the
agreement, our collaborative partners have the right to terminate the agreement
on short notice without cause. The termination or non-renewal of any
collaborative relationship could impede our research and development efforts.

         CONSOLIDATIONS AMONG COMPANIES WITH WHICH WE ARE ENGAGED IN
COLLABORATIVE RESEARCH CAN RESULT IN THE DIMINUTION OR TERMINATION OF, OR DELAYS
IN, ONE OR MORE OF OUR COLLABORATIVE PROGRAMS.

         In 1995, the pharmaceutical operations of three companies with which we
had collaborative research agreements, Hoechst AG, Hoechst Roussel
Pharmaceuticals, Inc. and Marion Merrell Dow Inc., were combined into one
entity. This combination resulted in delays in our collaborative programs with
each of the constituent companies and a reduction in the aggregate funding
received by us. Continued consolidations among large pharmaceutical companies
could produce similar results.

         FAILURE TO ATTRACT, RETAIN AND MOTIVATE SKILLED PERSONNEL AND CULTIVATE
KEY ACADEMIC COLLABORATIONS WILL DELAY OUR PRODUCT DEVELOPMENT PROGRAMS AND
RESEARCH AND DEVELOPMENT EFFORTS.

         We are a small company with approximately 200 employees, and our
success depends on our continued ability to attract, retain and motivate highly
qualified management and scientific personnel and on our ability to develop and
maintain collaborative relationships with leading academic institutions and
scientists. In particular, our product development programs depend on our
ability to attract and retain highly skilled chemists and clinical development
personnel for whom competition is intense. The loss of any of these personnel,
in particular, Colin Goddard, our Chief Executive Officer, could prevent us from
achieving our research and development objectives. We do not know if we will be
able to attract, retain or motivate personnel.

         IF THE CONTINUING EFFORTS OF GOVERNMENT AND THIRD-PARTY PAYORS TO
CONTAIN OR REDUCE THE COSTS OF HEALTH CARE SUCCEED, THE PRICE THAT WE OR ANY OF
OUR COLLABORATIVE PARTNERS OR OTHER LICENSEES WILL RECEIVE FOR ANY DRUGS
DISCOVERED OR DEVELOPED IN THE FUTURE MAY DECREASE SIGNIFICANTLY.

         In foreign markets, pricing and profitability of prescription
pharmaceuticals are subject to government control. In the United States, we
expect a number of federal and state proposals to implement similar government
control. In addition, increasing emphasis on managed care in the United States
will continue to put pressure on the pricing of pharmaceutical products. To the
extent that cost control initiatives have a negative effect on our collaborative
partners, our ability to commercialize our products and to realize royalties may
also be negatively affected.

         IF WE OR OUR COLLABORATIVE PARTNERS ARE REQUIRED TO OBTAIN LICENSES
FROM OTHERS, OUR ROYALTIES ON ANY COMMERCIALIZED PRODUCTS COULD BE REDUCED.

         The extent to which efforts by other researchers have resulted or will
result in patents and the extent to which we or our collaborative partners are
forced to obtain licenses from others, if available, is currently unknown. In
the case where we or our collaborative partners must obtain licenses from

                                      -6-
<PAGE>   10

third parties, fees must be paid for such licenses. These fees would reduce the
royalties we may receive on commercialized products.

         THE USE OF ANY OF OUR POTENTIAL PRODUCTS IN CLINICAL TRIALS AND THE
SALE OF ANY APPROVED PRODUCTS MAY EXPOSE US TO LIABILITY CLAIMS RESULTING FROM
THE USE OF PRODUCTS OR PRODUCT CANDIDATES.

         Consumers, pharmaceutical companies including our collaborative
partners, or others may make product liability claims against us. We do not
independently maintain product liability insurance coverage for claims arising
from the use of our products in clinical trials. Insurance coverage is becoming
increasingly expensive, and no assurance can be given that we will be a named
insured with respect to trials underway by our collaborative partners or others
or obtain insurance in the future at a reasonable cost or in sufficient amounts
to protect us. Our inability to obtain adequate liability insurance, or a
successful product liability claim or series of claims brought against us, could
negatively impact our business, financial condition and results of operations.

         IF OUR COMPETITORS SUCCEED IN DEVELOPING PRODUCTS THAT ARE MORE
EFFECTIVE THAN OUR OWN, OUR PRODUCTS MAY BE RENDERED OBSOLETE OR NONCOMPETITIVE.

         We face significant competition from industry participants who are
pursuing the same technologies as we and from organizations that are pursuing
pharmaceutical products that are competitive with our potential products. Most
of the organizations competing with us have greater capital resources, larger
research and development staffs and facilities, and more extensive experience in
drug discovery and development, obtaining regulatory approval and pharmaceutical
product manufacturing and marketing than we do. Our major competitors include
fully integrated pharmaceutical companies, such as Merck & Co., Inc., Glaxo
Wellcome Inc. and SmithKline Beecham plc, that have extensive drug discovery
efforts and are developing novel small molecule pharmaceuticals, as well as
numerous smaller companies. Companies pursuing different but related fields also
present significant competition for us. For example, research efforts with
respect to gene sequencing and mapping are identifying new and potentially
superior target genes.

         IF OUR COMPETITORS SUCCEED IN DEVELOPING TECHNOLOGIES THAT ARE MORE
EFFECTIVE THAN OUR OWN, OUR TECHNOLOGIES OR PRODUCTS PRODUCED THEREFROM MAY BE
RENDERED OBSOLETE OR NONCOMPETITIVE.

         Biotechnology and related pharmaceutical technology have undergone
rapid and significant change. We expect that the technology associated with our
research and development programs will continue to develop rapidly. Our future
success will depend in large part on our ability to maintain a competitive
position with respect to this technology. Rapid technological development by us
or others may result in compounds, products or processes becoming obsolete
before we recover any expenses incurred to develop a compound, product or
process.

         UNTIL REGISTERED UNDER THE REGISTRATION STATEMENT, THE SHARES WILL BE
RESTRICTED SECURITIES UNDER FEDERAL AND APPLICABLE STATE SECURITIES LAWS AND, AS
SUCH, MAY NOT BE TRANSFERRED, SOLD OR OTHERWISE DISPOSED OF, EXCEPT AS PERMITTED
UNDER SUCH SECURITIES LAWS OR PURSUANT TO REGISTRATION THEREUNDER OR AN
EXEMPTION THEREFROM.

         Prospective investors should be prepared to hold, and bear the economic
risk of an investment in, the Shares for an indefinite period. In addition, an
Investor should be able to withstand a total loss of its investment. As a
result, under certain circumstances, the ability of Investors to register the
common stock may be delayed.


                                      -7-
<PAGE>   11
         THE MARKET PRICES FOR SECURITIES OF BIOTECHNOLOGY AND PHARMACEUTICAL
COMPANIES HAVE HISTORICALLY BEEN HIGHLY VOLATILE, AND THE MARKET HAS FROM TIME
TO TIME EXPERIENCED SIGNIFICANT PRICE AND VOLUME FLUCTUATIONS THAT ARE UNRELATED
TO THE OPERATING PERFORMANCE OF PARTICULAR COMPANIES.

         The following factors may have a negative effect on our stock price:

         -        fluctuations in operating results;

         -        announcements of technological innovations or new therapeutic
                  products by others;

         -        clinical trial results;

         -        developments concerning strategic alliance agreements;

         -        government regulation;

         -        developments in patent or other proprietary rights;

         -        public concern as to the safety of our drugs;

         -        future sales of substantial amounts of our common stock by
                  existing stockholders; and

         -        comments by securities analysts and general market conditions.

         The realization of any of the risks described in these "Risk Factors"
could cause our stock price to fall dramatically.

         OUR CORPORATE GOVERNANCE DOCUMENTS, AND STATE LAW, PROVIDE CERTAIN
ANTI-TAKEOVER MEASURES WHICH WILL DISCOURAGE CERTAIN TYPES OF TRANSACTIONS
INVOLVING AN ACTUAL OR POTENTIAL CHANGE IN CONTROL OF THE COMPANY.

         Under our Certificate of Incorporation, our Board of Directors has the
authority, without further action by the stockholders, to fix the rights and
preferences of, and issue shares of, Preferred Stock. In January 1999, we
adopted a Shareholders Rights Plan, commonly referred to as a "Poison Pill."
Further, we are subject to Section 203 of the Delaware General Corporation Law
which, subject to certain exceptions, restricts certain transactions and
business combinations between a corporation and a stockholder owning 15% or more
of the corporation's outstanding voting stock for a period of three years from
the date the stockholder becomes an interested stockholder.


                                      -8-
<PAGE>   12
                                  RECENT EVENTS

     On February 28, 2000, OSI sold a total of 3,325,000 shares of common stock
to American Variable Insurance Series Global Small Capitalization Fund; Asset
Management Holding Co.; Biotechnology Value Fund, L.P.; Biotechnology Value Fund
II, L.P.; Chelsey Capital; Forstmann International Fund, LTD; Forstmann
Partners, L.P.; Galleon Healthcare Overseas, Ltd; Galleon Healthcare Partners,
LP; International Biotechnology Trust PLC; Investment 10, L.L.C.; Janus
Investment Fund; Lone Balsam, L.P.; Lone Cypress, LTD; Lone Sequoia, L.P.; Lone
Spruce, L.P.; Merlin BioMed LP; Merlin BioMed Intl LTD; Moore Global
Investments, Ltd.; Remington Investment Strategies, L.P.; Sands Point Partners;
SMALLCAP World Fund, Inc.; United Capital Management, Inc.; and Ziff Asset
Management, L.P. The gross proceeds of the sale were $56,525,000.00. OSI agreed
to file the registration statement, of which this prospectus is a part, which
enables the selling stockholders to sell their shares. OSI also agreed to
reimburse the selling stockholders for liability OSI causes by making any untrue
statements of material fact or failing to state a material fact in this
registration statement.


                                 USE OF PROCEEDS

         The proceeds from the sale of the shares of common stock are solely for
the account of the selling stockholders. OSI will not receive any proceeds from
the sale of the shares.


                              SELLING STOCKHOLDERS

         The table below describes the amount of common stock owned by the
selling stockholders on February 28, 2000 and the number of shares of common
stock the selling stockholders are selling under this prospectus.


<TABLE>
<CAPTION>
                                                                                   PERCENTAGE OF     PERCENTAGE OF
                                                                    SHARES          SHARES OWNED      SHARES OWNED
                                                  SHARES            OFFERED           PRIOR TO            AFTER
        SELLING STOCKHOLDERS                       OWNED            HEREBY           OFFERING(1)       CLOSING(1)
        --------------------                  ----------          ---------        --------------    -------------
<S>                                           <C>                 <C>              <C>               <C>
American Variable Insurance Series
Global Small Capitalization Fund                 14,000            14,000                *                 0

Asset Management Holding Co.                     13,000            13,000                *                 0

Biotechnology Value Fund, L.P.                  801,696            75,000                3.1%              2.8%

Biotechnology Value Fund, II L.P.             1,189,228           115,000                4.6%              4.2%

Chelsey Capital                                  50,000            50,000                *                 0

Forstmann International Fund, LTD                27,000            27,000                *                 0

Forstmann Partners, L.P.                         10,000            10,000                *                 0

Galleon Healthcare Overseas, Ltd                 71,500            71,500                *                 0
</TABLE>


                                      -9-
<PAGE>   13
<TABLE>
<S>                                           <C>                 <C>              <C>               <C>
Galleon Healthcare Partners, LP                  28,500            28,500                *                 0

International Biotechnology Trust PLC           500,000           500,000                2.0%              0

Investment 10, L.L.C.                            89,337            10,000                *                 *

Janus Investment Fund                         1,855,240         1,000,000                7.2%              3.3%

Lone Balsam, L.P.                                19,750            19,750                *                 0

Lone Cypress, LTD                               204,750           204,750                *                 0

Lone Sequoia, L.P.                               16,500            16,500                *                 0

Lone Spruce, L.P.                                 9,000             9,000                *                 0

Merlin BioMed LP                                200,000            50,000                *                 *

Merlin BioMed Intl LTD                          200,000            50,000                *                 *

Moore Global Investments, Ltd.                  200,100            80,000                *                 *

Remington Investment Strategies, L.P.            49,900            20,000                *                 *

Sands Point Partners                             50,000            50,000                *                 0

SMALLCAP World Fund, Inc.                     1,041,000           561,000                4.1%              1.9%

United Capital Management, Inc.                  50,000            50,000                *                 0

Ziff Asset Management, L.P.                     300,000           300,000                1.2%              0
</TABLE>


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

*    Less than 1.0%.

(1)  Based on shares of common stock outstanding as of February 28, 2000.


                                      -10-
<PAGE>   14
                              PLAN OF DISTRIBUTION

         Any distribution hereunder of the shares by the selling stockholders
may be effected from time to time in one or more of the following transactions:

         -        through brokers, acting as principal or agent, in transactions
                  (which may involve block transactions) on Nasdaq or otherwise,
                  in special offerings, in the over-the-counter market, or
                  otherwise, at market prices obtainable at the time of sale, at
                  prices related to such prevailing market prices, at negotiated
                  prices or at fixed prices,

         -        to underwriters who will acquire the shares for their own
                  account and resell them in one or more transactions, including
                  negotiated transactions, at a fixed public offering price or
                  at varying prices determined at the time of sale (any public
                  offering price and any discount or concessions allowed or
                  reallowed or paid to dealers may be changed from time to
                  time),

         -        directly or through brokers or agents in private sales at
                  negotiated prices,

         -        to lenders pledged as collateral to secure loans, credit or
                  other financing arrangements and any subsequent foreclosure,
                  if any, thereunder,

         -        through put or call options transactions relating to the
                  shares,

         -        through short sales of shares, or

         -        by any other legally available means.

         Also, offers to purchase shares may be solicited by agents designated
by the selling stockholders from time to time. Underwriters or other agents
participating in an offering made pursuant to this prospectus (as amended or
supplemented from time to time) may receive underwriting discounts and
commissions under the Securities Act of 1933, as amended, and discounts or
concessions may be allowed or reallowed or paid to dealers, and brokers or
agents participating in such transactions may receive brokerage or agent's
commissions or fees. The selling stockholders may effect sales of shares to or
through broker-dealers, and such broker-dealers may receive compensation in the
form of discounts, concessions or commissions from the selling stockholders
and/or the purchasers of the shares for whom such broker-dealers may act as
agents or to whom they sell as principals, or both (which compensation as to a
particular broker-dealer might be in excess of customary commissions).

         At the time a particular offering of any shares is made hereunder, to
the extent required by law, a prospectus supplement will be distributed which
will set forth the amount of shares being offered and the terms of the offering,
including the purchase price or public offering price, the name or names of any
underwriters, dealers or agents, the purchase price paid by any underwriter for
any shares purchased from the selling stockholders, any discounts, commissions
and other items constituting compensation from the selling stockholders and any
discounts, commissions or concessions allowed or filed or paid to dealers. The
shares may be sold from time to time in one or more transactions at a fixed
offering price, which may be changed, or at varying prices determined at the
time of sale or at negotiated prices. Such prices will be determined by the
selling stockholders or by agreement between the selling stockholders and
underwriters or dealers, if any. The selling stockholders also may, from


                                      -11-
<PAGE>   15
time to time, authorize dealers, acting as selling stockholders' agents, to
solicit offers to purchase the shares upon the terms and conditions set forth in
any prospectus supplement.

         In order to comply with the securities laws of certain states, if
applicable, the shares will be sold hereunder in such jurisdictions only through
registered or licensed brokers or dealers.

         OSI has been advised that, as of the date hereof, the selling
stockholders have made no arrangements with any broker for the sale of their
shares. The selling stockholders and any underwriters, brokers or dealers
involved in the sale of the shares may be considered "underwriters" as that term
is defined by the Securities Act although the selling stockholders disclaim such
status. OSI has agreed to reimburse the selling stockholders against certain
liabilities that may be incurred in connection with the sale of the shares under
this prospectus. In addition, the selling stockholders have agreed to reimburse
OSI against certain liabilities. OSI has agreed to pay certain expenses incident
to the registration statement and the sale of the shares hereunder to the
public, other than certain internal administrative and similar costs of the
selling stockholders, legal fees and expenses of counsel for the selling
stockholders and any underwriting discount and commissions, selling or placement
agent or broker fees or commissions, and transfer taxes, if any, in connection
with the sale of securities by the selling stockholders. OSI will not receive
any proceeds from any sales of the shares pursuant to this prospectus. Each
selling stockholder will be subject to applicable provisions of the Securities
Exchange Act of 1934, as amended, and the rules and regulations thereunder,
including, without limitation, Regulation M, which provisions may limit the
timing of purchases and sales of shares of OSI's common stock by the selling
stockholders.


                              AVAILABLE INFORMATION

         OSI has filed a registration statement, of which this prospectus is a
part, and related exhibits with the SEC pursuant to the Securities Act. The
registration statement contains additional information about OSI and OSI's
common stock. OSI also files annual and quarterly reports, proxy statements and
other information with the SEC. You may read and copy the registration statement
or any other document OSI files with the SEC at the SEC's Public Reference Room
at 450 Fifth Street, N.W., Washington, D.C. 20549. Please call the SEC at
1-800-SEC-0330 for further information on the Public Reference Room.

         The SEC also maintains a website that contains reports, proxy and
information statements, and other information that OSI has filed electronically.
The SEC's website is located at http://www.sec.gov. OSI also maintains its own
website which is located at http://www.osip.com.


                 INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

         The SEC allows OSI to "incorporate by reference" the information OSI
provides in documents filed with the SEC, which means that OSI can disclose
important information by referring to those documents. The information
incorporated by reference is an important part of this prospectus. Any statement
contained in a document which is incorporated by reference in this prospectus is
automatically updated and superseded if information contained in this
prospectus, or information that OSI later files with the SEC, modifies and
replaces this information. OSI incorporates by reference the following documents
OSI has filed with the SEC:


                                      -12-
<PAGE>   16
         1.       annual report on Form 10-K for the fiscal year ended September
                  30, 1999, filed with the SEC on December 29, 1999 and amended
                  on January 25, 2000;

         2.       current reports on Form 8-K filed with the SEC on December 15,
                  1999 and March 1, 2000;

         3.       quarterly report on Form 10-Q for the quarter ended December
                  31, 1999, filed with the SEC on February 14, 2000;

         4.       proxy statement, dated February 7, 2000, for OSI's 2000 annual
                  meeting of stockholders, filed with the SEC on January 28,
                  2000; and

         5.       The description of OSI's common stock, which is registered
                  under Section 12 of the Exchange Act, contained in OSI's
                  registration statement on Form 8-A, including any amendments
                  or reports filed for the purpose of updating such description.

         All documents OSI has filed with the SEC under Sections 13(a), 13(c),
14 or 15(d) of the Exchange Act after the date of this prospectus will become a
part of this prospectus. To receive a free copy of any of the documents
incorporated by reference in this prospectus call or write Robert L. Van
Nostrand, Vice President and Chief Financial Officer, OSI Pharmaceuticals, Inc.,
106 Charles Lindbergh Boulevard, Uniondale, New York 11553, telephone (516)
222-0023. OSI will not send exhibits to the documents unless those exhibits have
been specifically incorporated by reference in this prospectus.

         You should rely only on the information incorporated by reference or
included in this prospectus or the applicable prospectus supplement. OSI has not
authorized anyone else to provide you with different information. The selling
stockholders may only use this prospectus to sell securities if a prospectus
supplement is delivered with the prospectus, to the extent one is required. The
selling stockholders are only offering these securities in states where the
offer is permitted. You should not assume that the information in this
prospectus or the applicable prospectus supplement is accurate as of any date
other than the dates set forth on the front of these documents.


                                  LEGAL MATTERS

         Saul, Ewing, Remick & Saul LLP, Philadelphia, Pennsylvania, will pass
upon the validity of the shares of common stock offered in this prospectus for
OSI.


                                     EXPERTS

         The consolidated financial statements of OSI as of September 30, 1999
and 1998, and for each of the years in the three-year period then ended included
in OSI's 1999 annual report on Form 10-K, have been incorporated by reference in
this prospectus and in the registration statement in reliance upon the report of
KPMG LLP, independent certified public accountants, also incorporated by
reference in this prospectus, and upon the authority of KPMG LLP as experts in
accounting and auditing.


                                      -13-
<PAGE>   17
         You should rely only on the information contained in this prospectus or
incorporated by reference. OSI has not authorized anyone to provide you with
additional or different information. OSI is not making an offer of these
securities in any jurisdiction where the offer or sale is not permitted. You
should not assume that the information contained in or incorporated by reference
in this prospectus is accurate as of any date other than the date on the front
cover of this prospectus, regardless of the date of delivery of this prospectus
or the date of any sale of the securities.






                                3,325,000 shares

                            OSI PHARMACEUTICALS, INC.


                                  Common Stock

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

                                   Prospectus
                               ------------------



                                  March 2, 2000
<PAGE>   18
                                     PART II


               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

         The following expenses incurred in connection with the sale of the
securities being registered will be borne by OSI. Other than the SEC
registration fee, the amounts stated are estimates.

<TABLE>
<S>                                                                                   <C>
                  SEC registration fee............................................    $      19,777.93
                  Accounting fees and expenses....................................            5,000.00
                  Legal fees and expenses.........................................           10,000.00
                                                                                          ------------
                      Total.......................................................    $      34,777.93
</TABLE>

ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

         Section 145 of the General Corporation Law of Delaware empowers a
corporation to indemnify any person who was or is a party or is threatened to be
made a party to any threatened, pending or completed action, suit or proceeding,
whether civil, criminal, administrative or investigative, by reason of the fact
that he or she is or was a director, officer, employee or agent of the
corporation or another enterprise if serving such enterprise at the request of
the corporation. Depending on the character of the proceeding, a corporation may
indemnify against expenses (including attorney's fees), judgments, fines and
amounts paid in settlement actually and reasonably incurred in connection with
such action, suit or proceeding if the person indemnified acted in good faith
and in a manner he or she reasonably believed to be in or not opposed to the
best interests of the corporation, and, with respect to any criminal action or
proceeding, had no reasonable cause to believe his or her conduct was unlawful.
In the case of an action by or in the right of the corporation, no
indemnification may be made in respect to any claim, issue or matter as to which
such person shall have been adjudged to be liable to the corporation unless and
only to the extent that the Court of Chancery or the court in which such action
or suit was brought shall determine that despite the adjudication of liability,
but in view of all the circumstances of the case, such person is fairly and
reasonably entitled to indemnity for such expenses that the court shall deem
proper. Section 145 further provides that to the extent a director or officer of
a corporation has been successful in the defense of any action, suit or
proceeding referred to above, or in defense of any claim, issue or matter
therein, he or she shall be indemnified against expenses (including attorney's
fees) actually and reasonably incurred by him or her in connection therewith.

         OSI's Certificate of Incorporation provides that OSI shall, to the
fullest extent authorized by the General Corporation Law of Delaware, indemnify
any person, or the legal representative of any person, who is or was a director,
officer, employee or agent of OSI or another enterprise if said person served
such enterprise at the request of OSI. The Certificate of Incorporation also
provides that any amendment to the General Corporation Law of Delaware shall
only be applicable to the extent any such amendment permits OSI to provide
broader indemnification rights than said law permitted OSI to provide prior to
such amendment. The Certificate of Incorporation further provides that in the
case of an action, suit or proceeding initiated by the indemnified person, OSI
shall indemnify the person only if such action, suit or proceeding was
authorized by OSI's Board of Directors. The Certificate of Incorporation also
contains a provision eliminating the liability of directors of OSI to OSI or its


                                      II-1
<PAGE>   19
stockholders for monetary damages for breach of fiduciary duty except under
certain specified circumstances. The Certificate of Incorporation also permits
OSI to maintain insurance to protect itself and any director, officer, employee
or agent against any liability with respect to which OSI would have the power to
indemnify such persons under the General Corporation Law of Delaware. OSI
maintains an insurance policy insuring its directors and officers against
certain liabilities.

ITEM 16. EXHIBITS.

         The following is a list of exhibits filed as part of the Registration
Statement:


         4.1      Stock Purchase Agreement by and between OSI and Biotechnology
                  Value Fund, L.P., dated February 24, 2000 (included as an
                  exhibit to OSI's current report on Form 8-K filed on March 1,
                  2000). Each of the selling stockholders have executed Stock
                  Purchase Agreements which are substantially identical to
                  Biotechnology Value Fund L.P.'s Stock Purchase Agreement
                  except as to the parties and the number of shares purchased.

<TABLE>
<CAPTION>
                                                       Number of
           Party                                    Shares Purchased
           -----                                    ----------------
<S>                                              <C>
    American Variable Insurance Series
    Global Small Capitalization Fund                      14,000

    Asset Management Holding Co.                          13,000

    Biotechnology Value Fund, II L.P.                    115,000

    Chelsey Capital                                       50,000

    Forstmann International Fund, LTD                     27,000

    Forstmann Partners, L.P.                              10,000

    Galleon Healthcare Overseas, Ltd                      71,500

    Galleon Healthcare Partners, LP                       28,500

    International Biotechnology Trust PLC                500,000

    Investment 10, L.L.C.                                 10,000

    Janus Investment Fund                              1,000,000

    Lone Balsam, L.P.                                     19,750

    Lone Cypress, LTD                                    204,750

    Lone Sequoia, L.P.                                    16,500

    Lone Spruce, L.P.                                      9,000

    Merlin BioMed LP                                      50,000

    Merlin BioMed Intl LTD                                50,000

    Moore Global Investments, Ltd.                        80,000

    Remington Investment Strategies, L.P.                 20,000

    Sands Point Partners                                  50,000

    SMALLCAP World Fund, Inc.                            561,000

    United Capital Management, Inc.                       50,000

    Ziff Asset Management, L.P.                          300,000
</TABLE>

         5.1      Opinion of Saul, Ewing, Remick & Saul LLP as to the legality
                  of the securities registered hereunder

         23.1     Consent of KPMG LLP

         23.2     Consent of Saul, Ewing, Remick & Saul LLP (included in Exhibit
                  5.1)

         24.1     Power of Attorney (contained on Signature Page)

ITEM 17. UNDERTAKINGS.

         A.       Rule 415 Offering

                  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;

         Provided, however, that paragraphs (1)(i) and (1)(ii) above do not
apply if the registration statement is on Form S-3 or Form S-8, and the
information required to be included in a post-effective


                                      II-2

<PAGE>   20
amendment by those paragraphs is contained in periodic reports filed by the
registrant pursuant to section 13 or section 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. Filing Incorporating Subsequent Exchange Act Documents By Reference.

                  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 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.

         C. Request for Acceleration of Effective Date

                  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 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.


                                      II-3
<PAGE>   21
                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, as amended,
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 Uniondale, State of New York, on March 2, 2000.

                                       OSI PHARMACEUTICALS, INC.


                                       By: /s/ Colin Goddard
                                           -------------------------------------
                                           Colin Goddard, Ph.D.
                                           President and Chief Executive Officer

                                POWER OF ATTORNEY

         KNOW ALL MEN BY THESE PRESENTS, that each person whose signature
appears below hereby makes, constitutes and appoints Colin Goddard, Ph.D. and
Robert L. Van Nostrand, and each of them, with full power to act without the
other, his true and lawful attorney-in-fact and agent, with full power of
substitution and resubstitution, for him and in his name, place and stead, in
any and all capacities to sign any and all amendments to this Registration
Statement, including post-effective amendments, and to file the same, with all
exhibits thereto, and other documents in connection therewith, with the
Securities and Exchange Commission, granting unto said attorneys-in-fact and
agents, and each of them, full power and authority to do and perform each and
every act and thing requisite or necessary to be done in connection therewith,
as fully to all intents and purposes as he might or could do in person, hereby
ratifying and confirming all that said attorneys-in-fact and agents or any of
them, or any substitute or substitutes, may lawfully do or cause to be done by
virtue hereof.

         Pursuant to the requirements of the Securities Act of 1933, as amended,
this Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.


<TABLE>
<CAPTION>
          Signature                                                Title                           Date
          ---------                                                -----                           ----
<S>                                                      <C>                                     <C>
/s/ COLIN GODDARD, Ph.D.                                       President and                     March 2, 2000
- -----------------------------------
Colin Goddard, Ph.D.                                      Chief Executive Officer
                                                               and Director


/s/ ROBERT L. VAN NOSTRAND                               Vice President and Chief                March 2, 2000
- -----------------------------------
Robert L. Van Nostrand                                       Financial Officer


/s/ GARY E. FRASHIER                                            Chairman of                      March 2, 2000
- -----------------------------------
Gary E. Frashier                                          the Board of Directors


/s/ G. MORGAN BROWNE                                             Director                        March 2, 2000
- -----------------------------------
G. Morgan Browne
</TABLE>
<PAGE>   22
<TABLE>
<S>                                                      <C>                                     <C>
/s/ JOHN H. FRENCH, II                                           Director                        March 2, 2000
- -----------------------------------
John H. French, II


/s/ EDWIN A. GEE, Ph.D                                           Director                        March 2, 2000
- -----------------------------------
Edwin A. Gee, Ph.D.


/s/ DARYL K. GRANNER, M.D.                                       Director                        March 2, 2000
- -----------------------------------
Daryl K. Granner, M.D.


/s/ WALTER M. LOVENBERG, Ph.D.                                   Director                        March 2, 2000
- -----------------------------------
Walter M. Lovenberg, Ph.D.


/s/ STEVEN M. PELTZMAN                                           Director                        March 2, 2000
- -----------------------------------
Steven M. Peltzman


                                                                 Director                               , 2000
- -----------------------------------
Viren Mehta


 /s/ JOHN P. WHITE, Esquire                                      Director                        March 2, 2000
- -----------------------------------
John P. White, Esquire
</TABLE>
<PAGE>   23
                                  EXHIBIT INDEX

<TABLE>
<CAPTION>
    EXHIBIT NO.                             EXHIBIT                                      SEQUENTIAL PAGE NUMBER
    -----------                             -------                                      ----------------------
<S>                        <C>
      4.1                  Stock Purchase Agreement by and between OSI
                           and Biotechnology Value Fund L.P., dated
                           February 24, 2000 (included as an exhibit to
                           OSI's current report on Form 8-K filed on
                           March 1, 2000).

      5.1                  Opinion of Saul, Ewing, Remick & Saul LLP
                           as to the legality of the securities registered
                           hereunder

      23.1                 Consent of KPMG LLP

      23.2                 Consent of Saul, Ewing, Remick & Saul LLP
                           (included in Exhibit 5.1)

      24.1                 Power of Attorney (contained on Signature Page)
</TABLE>

<PAGE>   1
                                                                     EXHIBIT 5.1

                 [LETTERHEAD OF SAUL, EWING, REMICK & SAUL LLP]


                                                                   March 2, 2000


OSI Pharmaceuticals, Inc.
106 Charles Lindbergh Boulevard
Uniondale, NY  11553

Ladies and Gentlemen:

         We refer to the Registration Statement on Form S-3 (collectively with
any amendments thereto, the "Registration Statement") of OSI Pharmaceuticals,
Inc., a Delaware corporation (the "Company"), filed with the Securities and
Exchange Commission covering the registration under the Securities Act of 1933,
as amended (the "Securities Act"), of up to 3,325,000 shares of common stock,
par value $.01 per share, of the Company (the "Shares"), all as described in the
Prospectus included in the Registration Statement.

         We have examined the Registration Statement, the Certificate of
Incorporation and the Amended and Restated By-laws of the Company and such
records, certificates and other documents as we have considered necessary or
appropriate for the purposes of this opinion. Without limiting the generality of
the foregoing, in our examination we have assumed without independent
verification that (i) each natural person executing a document we examined is
legally competent to do so, (ii) all documents submitted to us as originals are
"authentic", the signatures on all documents that we examined are genuine, and
all documents submitted to us as certified, conformed, photostatic or facsimile
copies conformed to the original document, and (iii) all corporate records made
available to us by the Company and or public records reviewed are accurate and
complete.

         Based on the foregoing, it is our opinion that the Shares were, when
issued, duly authorized, validly issued, fully paid and non-assessable.

         We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the Prospectus contained therein. In giving this consent, we
do not admit that we are in the category of persons whose consent is required
under Section 7 of the Securities Act or the rules and regulations of the
Securities and Exchange Commission thereunder.

                                         Very truly yours,


                                         /s/ SAUL, EWING, REMICK & SAUL LLP
                                         --------------------------------------

<PAGE>   1
                                                                    EXHIBIT 23.1

                         CONSENT OF INDEPENDENT AUDITORS


The Board of Directors
OSI Pharmaceuticals, Inc.

         We consent to the use of our report dated December 22, 1999,
incorporated herein by reference, and to the reference to our firm under the
heading "Experts" in the prospectus.


                                          /s/   KPMG LLP
                                          -------------------------------------

Melville, New York
February 28, 2000


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