Should the two groups merge as planned, the new entity would immediately have revenues of over $23 billion (£15.0 billion) and more than 40 per cent of the global ad market.
The announcement is planned for 2pm French time. Significant share price movement is expected when the markets open on Monday, both for Omnicom and Publicis, as well as for their rivals British group WPP and US-based Interpublic Group.
Publicis boss Maurice Levy is lined up to be the chairman of the new entity, with the younger Omnicom boss John Wren as the chief executive. Both spent much of today (Saturday) briefing their agency heads around the world in advance of tomorrow's confirmation of an intended merger.
As media regulators around the world begin to consider the merger, Omnicom and Publicis agency heads will begin looking at manifold potential client conflicts.
For example Omnicom-owned ad network BBDO produces the bulk of Pepsi's advertising, while Publicis-owned Leo Burnett handles Coca-Cola. Rival clients such as these will need to be reassured that they will be served by separate teams, and with the same levels of dedication and loyalty.
WPP, with current revenues of $15 billion, could now become the number two global player in marketing services. But led by the fiercely competitive Sir Martin Sorrell, we should expect a flurry of activity.
WPP agencies will seek to poach disaffected clients and staff. Sorrell may also consider a bid for Interpublic Group or French-owned Havas to boost his media-buying scale.
Scepticism remains among analysts and insiders about whether such a merger will work culturally, with the Waspish Omnicom forced together with the Gallic flair of Publicis, particularly as Wren and Levy will become joint-chief executives in reality.
Spokespeople for Omnicom and Publicis Groupe declined to comment.