Exhibit 5.1



                      [Letterhead of Dewey Ballantine LLP]



                                                     November 12, 2004


Omnicare, Inc.
100 East RiverCenter Blvd.
Suite 1600
Covington, Kentucky 41011

         Re:   Omnicare, Inc. Registration Statement on Form S-8
               -------------------------------------------------

Ladies and Gentlemen:

         We have acted as counsel for Omnicare, Inc., a Delaware corporation
("Omnicare"), in connection with the registration by Omnicare under the
Securities Act of 1933, as amended (the "Act"), of 9,974,652 shares of common
stock, par value $1.00 per share (the "Shares"), issued under the Company's 2004
Stock and Incentive Plan (the "Plan"). The Shares are to be registered under the
Registration Statement on Form S-8 filed with the Securities and Exchange
Commission today (the "Registration Statement").

         We are familiar with the proceedings of Omnicare relating to the
authorization and issuance of the Shares. In addition, we have made such further
examinations of law and fact as we have deemed appropriate in connection with
the opinion hereinafter set forth. We express no opinion as to the law of any
jurisdiction other than the laws of the State of New York and the corporate laws
of the State of Delaware.

         Based upon the foregoing, we are of the opinion that the Shares have
been duly authorized and, when issued, 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. In giving such consent, we do not hereby admit that we
come within the category of persons whose consent is required under Section 7 of
the Act, or under the rules and regulations of the Securities and Exchange
Commission.


                                                     Very truly yours,

                                                     /s/ Dewey Ballantine LLP

                                                     Dewey Ballantine LLP