Zed won the business following a four-way pitch against MEC Interaction, Universal McCann and the incumbent, Green Cathedral.
The agency already handles the offline activity for the OU, which appointed Zed to integrate online and offline media channels.
Suzy McGill, the head of acquisition marketing at the OU, said: "As consumer media consumption has changed, we have moved towards more integrated multimedia planning. Zed is well placed to provide this service to the OU."
Kevin Murphy, the board planning director at Zed, said: "We have a long-standing relationship with the OU, but the result was anything but a foregone conclusion, so we are doubly pleased to have been appointed."The OU has around 150,000 undergraduate students and 30,000 postgraduates. The vast majority study part-time while in full-time employment.