The pitch, which was held through Oystercatchers, was called after the weight loss programme parted ways with the incumbent, Publicis Groupe’s Razorfish. Razorfish had held the account for seven years but decided not to repitch.
OMD, which also holds Weight Watchers offline media account, will now provide digital buying and analytics for the diet brand.
Saatchi & Saatchi, which works on the Weight Watchers creative account, is understood to have added the digital creative business to its remit after partnering with OMD.
Weight Watchers said it was not yet in a position to comment on the pitch result. OMD also declined to confirm whether it had won the account.
Weight Watchers was founded in the US in the 1960s and aims to help members lose weight through support meetings, branded foods and its trademark "ProPoints" system, which measures the nutritional content of food and restricts dieters to a certain amount.
The brand's online presence includes a website with tracking tools and a database of more than 30,000 kinds of food.