The group, which owns media agencies OMD and PHD, is set to place millions of dollars' worth of display ads for its clients, which include PepsiCo, McDonald's, Apple and Sony, on Google sites over the next few years.
In exchange, Google will work to create a "global trading desk" that will allow the advertising network to help Omnicom purchase ads more easily on Google’s ad exchange.
The internet firm, which also owns video content site YouTube, will then supply Omnicom with pinpoint data and research on how well the advertising has worked.
Any deal is understood to be non-binding, meaning that Google is allowed to work with other advertising networks; both WPP and Publicis group have long-standing arrangements.
Omnicom will also have a non-exclusivity deal, which will allow it to trade freely with other media owners, including Google’s rivals Yahoo! and Facebook.
A spokeswoman for Omnicom Media Group, said: "Google will help co-develop OMG’s display buying capabilities, including their trading desk, which buys display ad space on exchanges including the DoubleClick Ad Exchange.
"We are currently buying tens of millions each year through Trading Desk, and because we have every expectation that the number will continue to rise due to client success and interest, we are locking into best of breed deals like the Google agreement.
"As part of our new agreement with Google, our clients will also have access to attractive display pricing on Google's "owned" properties including YouTube."
OMG insisted that the deal would not include "pre-committing client dollars."
WPP, which owns media agencies MediaCom, Mediaedge:cia, Mindshare and Maxus, still spends more than any other any network on Google sites across the world, and the Omnicom deal is not expected to top that.
Google is understood to be looking to diversify its model away from being perceived as just another search engine.
However, it faces a challenge with the decline of online web display advertising, with Europe alone experiencing flat year-on-year figures in the display market, with an actual decline in mature European markets.
The UK was down by close to 5% year on year, France was down 6% and Sweden was down 5%, according to the most recent figures from the Interactive Advertising Bureau Europe, released at the beginning of last month.
Picture credit: Nigel Parry