Exhibit 99.1

                                POWER OF ATTORNEY

      KNOW ALL MEN BY THESE PRESENTS, that the undersigned hereby constitutes
and appoints Barry J. Wagner, and any other duly appointed Secretary or
Assistant Secretary of Omnicom Group Inc., with full power of substitution, as
the undersigned's true and lawful authorized representative and attorney-in-fact
to:

      (1)   execute for and on behalf of the undersigned, in the undersigned's
            capacity as an officer or director of Omnicom Group Inc. (the
            "Company"), Forms 3, 4, and 5, and any and all amendments thereto,
            in accordance with Section 16 of the Securities and Exchange Act of
            1934, as amended (the "1934 Act"), and the rules and regulations
            promulgated thereunder;

      (2)   do and perform any and all acts for and on behalf of the undersigned
            which may be necessary or desirable to complete the execution of any
            such form or schedule and the timely filing of such form or schedule
            with the United State Securities and Exchange Commission and any
            stock exchange or stock Market or other authority; and

      (3)   take any other action of any type whatsoever in connection with the
            foregoing which, in the opinion of such attorney-in-fact, may be of
            benefit to, in the best interest of, or legally required by, the
            undersigned, it being understood that the documents executed by such
            attorney-in-fact on behalf of the undersigned pursuant to this Power
            of Attorney shall be in such form and shall contain such terms and
            conditions as such attorney-in-fact may approve in such
            attorney-in-fact's discretion.

      The undersigned hereby grants to such attorney-in-fact full power and
authority to do and perform all and every act and thing whatsoever requisite,
necessary or proper to be done in the exercise of any of the rights and powers
herein granted, as fully to all intents and purposes as the undersigned might or
could do if personally present, with full power of substitution or revocation,
hereby ratifying and confirming all that such attorney-in-fact, or such
attorney-in fact's substitute shall lawfully do or cause to be done by virtue of
this power of attorney and the rights and powers herein granted. The undersigned
acknowledges that the foregoing attorney-in-fact, in service in such capacity at
the request of the undersigned, is not assuming, nor is the Company assuming,
any of the undersigned's responsibilities to comply with Section 16 or any other
provision of the 1934 Act.

      This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4 and 5 with respect to the
undersigned's holding of transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
foregoing attorneys-in-fact.



      IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 16th day of September, 2003.

                                                          /s/ Errol M. Cook
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                                                          Signature

                                                          Errol M. Cook
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                                                          Print Name


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