The group, which owns agency networks including BBDO, TBWA, DDB, OMD and PHD, saw revenue in its worldwide operations fall 14.4% to $2.8bn (£1.69bn) in the three months to 30 September.
Within the US, the New York-based group saw revenue fall 13.2% to $1.5bn (£905m), while international revenue decreased 15.8% to $1.35bn (£815m).
Last year, Omnicom announced plans to cut almost 5% of its workforce around the world, equalling up to 3,500 jobs, in a bid to cope with the recession.
In April, the group reported that net income dropped 21.2% to $164.5m (£112.2m) during the first quarter.
In other results, WPP, owner of JWT, MindShare and Hill & Knowlton, recently reported a pre-tax profit fall of 47% to £179.3m in the six months to 30 June.