OMD, which has held the main business since the channel's launch in 1982, fended off competition from ZenithOptimedia for the account in the final pitch.
Channel 4's media responsibilities had previously been shared among OMD, with Profero holding the online planning and buying role, and Michaelides and Bednash (M&B) used for comms planning.
Channel 4 considered keeping its online planning and buying separate, with incumbent Profero invited to pitch for the channel's online marketing requirements.
But is understood the broadcaster preferred a "one-stop shop" approach, and was satisfied that OMD could develop its digital activities after the agency made additional presentations about its plans for the business.
The pitch kicked off in May, with Channel 4 drawing up a shortlist of agencies.
M&B will continue to work for the channel on a project basis, including the launch of E4 Radio.