The marketing services company reports net income for the three months to 31 December fell to $229.6m (£147m) from $298.5m (£191.2m) a year earlier.
Profit was dented by a 3.1% fall in revenue to $3.2bn (£2bn) from $3.37 (£2.1bn) in the fourth quarter of 2008.
The US group was hit by a 7.3% decline in domestic revenues, partly offset by a 1.5% increase in international revenue, which includes monies earned in the UK.
The company says full-year net income decreased 20.7% to $793.0m (£508m) on revenues of £11.7bn (£7.5bn), down 12.3% on the previous 12 months.
Omnicom’s results follows fourth quarter figures from Euro RSCG and MPG owner Havas. The French group posted a 4% decline in group revenues for the fourth quarter and a 7.9% decline for 2009.