Exhibit 5.1

                         [Cooley Godward LLP Letterhead]

November 13, 2003

Onyx Pharmaceuticals, Inc.
3031 Research Drive
Richmond, CA 94806

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Onyx Pharmaceuticals, Inc. (the "Company"), a Delaware
corporation, of a Registration Statement on Form S-8 (the "Registration
Statement") with the Securities and Exchange Commission covering the offering of
up to 700,000 shares of the Company's Common Stock, par value $0.001 per share
(the "Shares"), of which 600,000 shares are issuable pursuant to the Company's
1996 Equity Incentive Plan, as amended (the "1996 Plan") and 100,000 shares are
issuable pursuant to the 1996 Non-Employee Directors' Plan, as amended (the
"Directors' Plan").

In connection with this opinion, we have examined the Registration Statement and
related Prospectuses, your Amended and Restated Certificate of Incorporation and
Bylaws, and such other documents, records, certificates, memoranda and other
instruments as we deem necessary as a basis for this opinion. We have assumed
the genuineness and authenticity of all documents submitted to us as originals,
the conformity to originals of all documents submitted to us as copies thereof,
and the due execution and delivery of all documents where due execution and
delivery are a prerequisite to the effectiveness thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when sold and issued in accordance with (i) the 1996 Plan or
the Directors' Plan, as applicable, and (ii) the Registration Statement and
related Prospectuses, will be validly issued, fully paid, and nonassessable
(except as to shares issued pursuant to certain deferred payment arrangements,
which will be fully paid and nonassessable when such deferred payments are made
in full).

We consent to the filing of this opinion as an exhibit to the Registration
Statement.

Very truly yours,

COOLEY GODWARD LLP

    /s/ Robert L. Jones
By: ___________________________
    Robert L. Jones