As part of the deal, Omnicom's (OMC +0.2%) Accuen ad trading desk, which makes programmatic ad buys (a fast-growing market), will make purchases via Twitter's (TWTR +0.5%) MoPub mobile ad exchange/network.
In addition, Omnicom will be locking in ad rates and inventory access from Twitter, and get a "first look" at new Twitter ad products, of which there have been many as of late.
The deal follows an agreement reached last year with Starcom MediaVest that's reportedly worth $200M over 2 years. For reference, Twitter's 2014 and 2015 revenue consensus estimates are at $1.27B and $2.04B.
Twitter asserts MoPub's ad network reaches 1B Android/iOS users. Going forward, MoPub will also be used to deliver ads on Twitter's site and apps (e.g. Twitter's app install ads).