Previously, the business was split between The Allmond Partnership, which handled the bulk of the buying, PHD, which handled planning, and Zenith, which oversaw the press and radio buying.
TAP, PHD and Vizeum were invited to pitch when the review was launched in the autumn. Michaelides & Bednash was also involved in the final pitch.
Media Planning Group was ruled out at an earlier stage of the review.
The appointment of Zenith will consolidate both media planning and buying into one agency for the first time since O2 launched in 2001, as part of the newly created MmO2.
O2's media review follows an overhaul of the company's other agency relationships, which saw it hand its creative account to Vallance Carruthers Coleman Priest while Archibald Ingall Stretton won its £20 million direct brief at the beginning of last year.
Zenith declined to comment as Campaign went to press. O2 said that a result was imminent.