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 September 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 November 5, 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