For O&M, the move loosens the agency's grip on the Lucozade brand, which it has handled for the past 12 years. However, the agency will continue working on Lucozade Energy and other Glaxo SmithKline brands around the world.
For M&C Saatchi, the assignment extends a relationship with Lucozade's owner, Glaxo SmithKline. Two years ago, the agency ran advertising promoting the merger of Glaxo Wellcome and SmithKline Beecham to create the current company.
But Nick Hurrell, M&C Saatchi's joint chief executive, said: "Although our existing link with Glaxo SmithKline didn't do us any harm, it wasn't the reason for appointing us."
O&M repitched for Lucozade Sport against M&C Saatchi and TBWA/London.
The agency change is the result of intensified competition in the sports nutrition market. Coca-Cola relaunched Powerade in the UK two years ago, while PepsiCo is also stepping up its presence in the sector.
Hurrell said: "Lucozade Sport is doing very well in the category and our appointment is an attempt to make the advertising even more muscular."
New TV and print advertising for Lucozade Sport will appear later this year. Previous advertising has starred the England footballers Michael Owen and Alan Shearer.
But Hurrell said: "Our work will continue to feature sports people but won't be celebrity-focused."
Glaxo SmithKline this week ruled out any plan to review the Lucozade Energy brand for which O&M created a series of ads featuring Lara Croft of Tomb Raider fame.