The three networks will use GPlus as their 'preferred supplier' in Brussels. Omnicom's other major PR brand Fleishman-Hillard, which has its own Brussels office, will continue to operate separately.
Four GPlus staff including directors Peter Guilford and Nigel Gardner have been put on three-year earnouts, with an option to extend by
a further year.
Former Financial Times Brussels correspondent John Wyles and Micheal Tscherny, a competition spokesman under ex-EU commissioner Mario Monti, make up the earnout quartet.
The deal has been struck with Omnicom's non-advertising arm, Diversified Agency Services (DAS). It has acquired 84 per cent of the agency, with the remaining equity retained to incentivise other senior staff.
Guilford and Gardner founded GPlus in 2000, having previously held senior posts within the EU commission. The agency generates annual fee income of around £3.2m and employs 27 staff, including five in London.
It is understood that the performance-related deal could value the firm at £6m to £8m. GPlus's clients include Microsoft, Royal Bank of Scotland and Wal-Mart.
It will continue its informal strategic alliances with independent London PA consultancies, including Lexington Communications, Media Strategy, Portland PR and Luther Pendragon. Luther partners George Pitcher and Charles Stewart Smith have offloaded the minority stake in GPlus they inherited after their MBO last year.
DAS chief executive Anthony Wreford said: 'A European perspective is increasingly important in making business decisions.'