Exhibit 32.1

                                CERTIFICATION OF
                          QUARTERLY REPORT ON FORM 10-Q

      Pursuant  to 18 U.S.C.  ss.  1350,  as adopted  pursuant to ss. 906 of the
Sarbanes-Oxley  Act of 2002,  in  connection  with the filing of  Omnicom  Group
Inc.'s  Quarterly  Report on Form 10-Q for the quarter  ended June 30, 2004,  as
filed with the  Securities  and  Exchange  Commission  on the date  hereof  (the
"Report"),  each of the  undersigned  officers of Omnicom  Group Inc.  certifies
that, to such officer's knowledge:

o     the Report fully complies with the  requirements of Section 13(a) or 15(d)
      of the Securities Exchange Act of 1934; and

o     the information  contained in the Report fairly presents,  in all material
      aspects,  the  financial  condition  and results of  operations of Omnicom
      Group Inc. as of the dates and for the periods expressed in the Report.


Executed as of August 9, 2004.

                                                  /s/  JOHN D. WREN
                                           ------------------------------------
                                           Name:  John D. Wren
                                           Title: Chief Executive Officer and
                                                  President

                                                  /s/  RANDALL J. WEISENBURGER
                                           ------------------------------------
                                           Name:  Randall J. Weisenburger
                                           Title: Executive Vice President and
                                                  Chief Financial Officer