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                                                                     EXHIBIT 5.1

                                             
                                                                             lawyers@saul.com
SAUL EWING                                                                       www.saul.com



                                                                  April 22, 2005

OSI Pharmaceuticals, Inc.
58 South Service Road
Suite 110
Melville, NY 11747

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 84,940 shares of common stock, par value $.01
per share, of the Company (the "Shares"), as described in the Prospectus
included in the Registration Statement.

     We have examined the Registration Statement, the Certificate of
Incorporation, as amended, 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 LLP

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