The announcement throws into question Optimedia's tenure of the Tropicana brand, which is part of the Frito-Lay division alongside Walkers and Quaker.
The food giants have made the move as part of a realignment into Omnicom and did not invite the two agencies to conduct a formal pitch based on their media credentials.
Instead, the decision to move the accounts was made on the basis of cost tenders submitted to PepsiCo's procurement teams. But OMD's managing director, Steve Williams, said that the review process had been rigorous and the business had not been guaranteed to go OMD's way automatically.
Walkers is globally aligned on the creative side to Omnicom's BBDO network, with Abbott Mead Vickers BBDO having handled the UK business since 1998.
AMV continues to use Gary Lineker in its advertising, which was originally created by BMP DDB.
The decision to switch the UK media account into OMD comes one year after PepsiCo moved the entirety of its £371 million US broadcast business into OMD.
The majority of the £25 million UK media spend goes on Walkers' range of snack-food products. This year, Walkers launched the premium crisp brand Sensations in a challenge to Kettle Chips' hold on the market. The Quaker brands include Sugar Puffs, Harvest Crunch and Scotts porridge oats.
The news comes as a blow to MediaVest - the agency has produced memorable media campaigns for Walkers crisps and has played a part in its success.
Walkers achieved sales four times greater than that of its nearest rival Pringles, with 11 million packets of crisps sold daily. It has annual sales of £490 million.
Neil Campbell, Walkers' vice-president of marketing, said: "We operate in a very competitive market and have made our decision to move to OMD following a very rigorous review process."
OMD will begin working on the media account from the beginning of 2003.