OSI PHARMACEUTICALS INC
S-8 POS, 2000-03-03
IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES
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<PAGE>   1

      As filed with the Securities and Exchange Commission on March 3, 2000


                                                Registration No. 333-39509

- --------------------------------------------------------------------------------
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549


                        POST-EFFECTIVE AMENDMENT NO. 1 TO

                                    FORM S-8

                             REGISTRATION STATEMENT
                                      under
                           THE SECURITIES ACT OF 1933

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

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

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

                   Delaware                               13-3159796
       (State or other jurisdiction of                 (I.R.S. Employer
        incorporation or organization)                Identification No.)

             106 Charles Lindbergh Blvd.
                   Uniondale, NY                             11553
       (Address of principal executive offices)            (Zip Code)

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

                            OSI PHARMACEUTICALS, INC.
                        1997 INCENTIVE AND NON-QUALIFIED
                                STOCK OPTION PLAN
                            (Full title of the plan)

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

                             ROBERT L. VAN NOSTRAND
                   VICE PRESIDENT AND CHIEF FINANCIAL OFFICER

                            OSI PHARMACEUTICALS, INC.

             106 CHARLES LINDBERGH BLVD., UNIONDALE, NEW YORK 11553
                                 (516) 222-0023
            (Name, address, including zip code, and telephone number,
                   including area code, of agent for service)
                             -----------------------

                                    Copy to:

                         SPENCER W. FRANCK, JR., ESQUIRE
                         SAUL, EWING, REMICK & SAUL LLP

              1500 MARKET STREET, CENTRE SQUARE WEST, 38TH FLOOR,
                        PHILADELPHIA, PENNSYLVANIA 19102
                                 (215) 972-1955


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


    Registration fee previously paid with Form S-8 filed on November 4, 1997


                             -----------------------
<PAGE>   2




                                EXPLANATORY NOTE



         This Post-Effective Amendment No. 1 on Form S-8 is being filed to amend
the Form S-8 filed by OSI Pharmaceuticals, Inc. on November 4, 1997 in
connection with its 1997 Incentive and Non-Qualified Stock Option Plan. This
amendment is being filed to include a reoffer prospectus prepared in accordance
with the requirements of Part I of Form S-3 as well as to update information in
Part II.


<PAGE>   3
REOFFER PROSPECTUS

                                  22,222 SHARES

                            OSI PHARMACEUTICALS, INC.

                     COMMON STOCK, PAR VALUE $.01 PER SHARE


         The stockholder named on page 9 is selling up to 22,222 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 3, 2000


                                      -2-
<PAGE>   4
                                TABLE OF CONTENTS



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


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


SELLING STOCKHOLDER....................................................       9


PLAN OF DISTRIBUTION...................................................      10


AVAILABLE INFORMATION..................................................      11


INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE........................      11


LEGAL MATTERS..........................................................      12


EXPERTS................................................................      12
</TABLE>


                                      -i-
<PAGE>   5
                                  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>   6
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>   7
            -   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>   8
         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>   9
            -   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>   10
         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>   11
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>   12
         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>   13
                                 USE OF PROCEEDS

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


                               SELLING STOCKHOLDER

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


<TABLE>
<CAPTION>
                                                                                   PERCENTAGE OF     PERCENTAGE OF
                                                                    SHARES          SHARES OWNED      SHARES OWNED
                                                  SHARES            OFFERED           PRIOR TO            AFTER
         SELLING STOCKHOLDER                       OWNED            HEREBY           OFFERING(1)       CLOSING(1)
         -------------------                      ------            -------        -------------     -------------
<S>                                              <C>                <C>            <C>               <C>

          Arthur M. Bruskin                      202,414*           22,222             **                **
</TABLE>


- ----------

*   Includes 200,100 shares that may be acquired at or within 60 days of
    December 31, 1999 pursuant to the exercise of outstanding options.

**  Less than 1.0%.

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


                                      -9-
<PAGE>   14
                              PLAN OF DISTRIBUTION

         Any distribution hereunder of the shares by the selling stockholder 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 stockholder 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 stockholder 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 stockholder
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 stockholder, any discounts, commissions
and other items constituting compensation from the selling stockholder 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 stockholder or by agreement between the selling stockholder and
underwriters or dealers, if any. The selling stockholder also may, from


                                      -10-
<PAGE>   15
time to time, authorize dealers, acting as selling stockholder's 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.

         The selling stockholder 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 stockholder disclaim such
status. 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 stockholder, legal fees
and expenses of counsel for the selling stockholder 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 stockholder. 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 stockholder.


                              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:

         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;


                                      -11-
<PAGE>   16

         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 stockholder, 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
stockholder 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 stockholder 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.



                                      -12-
<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.






                                  22,222 shares

                            OSI PHARMACEUTICALS, INC.


                                  Common Stock

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

                                   Prospectus

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



                                  March 3, 2000


<PAGE>   18
                                     PART I
              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS


ITEM 1.           PLAN INFORMATION.(1)

ITEM 2.           REGISTRANT INFORMATION AND EMPLOYEE PLAN ANNUAL
                  INFORMATION.(1)


                                     PART II

               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT


ITEM 3.           INCORPORATION OF DOCUMENTS BY REFERENCE.

                  The documents listed in clauses (a), (b) and (c) below are
incorporated herein by this reference thereto, and all documents subsequently
filed by the Registrant pursuant to Sections 13(a), 13(c), 14 and 15(d) of the
Securities Exchange Act of 1934, prior to the filing of a post-effective
amendment which indicates that all securities offered have been sold or which
deregisters all securities then remaining unsold, shall be deemed to be
incorporated by this reference in this registration statement and to be a part
hereof from the date of filing of such documents:


         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 stockholder, 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.


ITEM 4.           DESCRIPTION OF SECURITIES.

                  Not applicable.

- ---------------------
(1)  The information called for by Part I of Form S-8 is currently included in
     the description of the Registrant's 1997 Incentive and Non-Qualified Stock
     Option Plan (the "Plan") delivered to eligible persons under the Plan.
     Pursuant to the Note to Part I of Form S-8, this information is not being
     filed with or included in this Form S-8.

                                      -1-
<PAGE>   19
ITEM 5.           INTERESTS OF NAMED EXPERTS AND COUNSEL.

                  Not applicable.

ITEM 6.           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
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 7.           EXEMPTION FROM REGISTRATION CLAIMED.

                  Not applicable.

ITEM 8.           EXHIBITS.

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

                                      -2-
<PAGE>   20

                  5        Opinion of Saul, Ewing, Remick & Saul LLP (filed
                           herewith)

                  23.1     Consent of KPMG LLP (filed herewith)

                  23.2     Consent of Saul, Ewing, Remick & Saul LLP (contained
                           in Exhibit No. 5).

                  24       Power of Attorney (previously filed).

                  99       1997 Incentive and Non-Qualified Stock Option Plan
                           (previously filed)


ITEM 9.           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 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-
<PAGE>   21
                  (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.


                                      -4-
<PAGE>   22
                                   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 Post-Effective Amendment No. 1 on Form S-8
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 3, 2000.



                                  OSI PHARMACEUTICALS, INC.



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




         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 3, 2000
- -----------------------------------        Chief Executive Officer
Colin Goddard, Ph.D.                            and Director


/s/ ROBERT L. VAN NOSTRAND                Vice President and Chief                March 3, 2000
- -----------------------------------           Financial Officer
Robert L. Van Nostrand


/s/ GARY E. FRASHIER*                            Chairman of                      March 3, 2000
- -----------------------------------        the Board of Directors
Gary E. Frashier


/s/ G. MORGAN BROWNE*                             Director                        March 3, 2000
- -----------------------------------
G. Morgan Browne
</TABLE>

<PAGE>   23

<TABLE>
<S>                                       <C>                                     <C>
/s/ JOHN H. FRENCH, II*                           Director                        March 3, 2000
- -----------------------------------
John H. French, II


/s/ EDWIN A. GEE, Ph.D*                           Director                        March 3, 2000
- -----------------------------------
Edwin A. Gee, Ph.D.


/s/ DARYL K. GRANNER, M.D.*                       Director                        March 3, 2000
- -----------------------------------
Daryl K. Granner, M.D.


/s/ WALTER M. LOVENBERG, Ph.D*.                   Director                        March 3, 2000
- -----------------------------------
Walter M. Lovenberg, Ph.D.


/s/ STEVEN M. PELTZMAN*                           Director                        March 3, 2000
- -----------------------------------
Steven M. Peltzman


                                                  Director                        March 3, 2000
- -----------------------------------
Viren Mehta


/s/ JOHN P. WHITE*                                Director                        March 3, 2000
- -----------------------------------
John P. White, Esquire
</TABLE>



                                               /s/ ROBERT L. VAN NOSTRAND
                                              ---------------------------------
                                          *By:     Robert L. Van Nostrand
                                                   Attorney-in-Fact





                                       2
<PAGE>   24
                                  EXHIBIT INDEX


<TABLE>
<CAPTION>
EXHIBIT NO.                               EXHIBIT

<S>              <C>
5                Opinion of Saul, Ewing, Remick & Saul LLP (filed herewith).

23.1             Consent of KPMG LLP (filed herewith).

23.2             Consent of Saul, Ewing, Remick & Saul LLP (Contained in
                   Exhibit No. 5).

25               Power of Attorney authorizing Gary E. Frashier and Robert
                 L. Van Nostrand to sign the Registration Statement
                 (previously filed).

99               1997 Incentive and Non-Qualified Stock Option Plan
                   (previously filed).
</TABLE>



<PAGE>   1
                                                                       EXHIBIT 5

                     [SAUL, EWING, REMICK & SAUL LETTERHEAD]

                                                                   March 3, 2000
OSI Pharmaceuticals, Inc.
106 Charles Lindbergh Blvd.
Uniondale, NY  11553

Gentlemen:

         We refer to the Registration Statement on Post-Effective Amendment No.
1 on Form S-8 (the "Registration Statement") of OSI Pharmaceuticals, Inc., a
Delaware corporation (the "Company"), to be filed with the Securities and
Exchange Commission covering the registration under the Securities Act of 1933,
as amended (the "Securities Act"), of 2,000,000 shares of common stock, par
value $.01 per share, of the Company (the "Shares"), which Shares are to be
issued under the Company's 1997 Incentive and Non-Qualified Stock Option Plan.

         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:

         1.       the Company is duly organized, validly existing and in good
standing under the laws of State of Delaware; and

         2.       the Shares to be issued in accordance with the terms described
in the Registration Statement have been duly authorized and, when issued in
accordance with the terms described in the Registration Statement, will be
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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