The WPP agency will work on creative and strategy for the brand, which wants to widen its remit and drive "social relevancy" throughout region.
Brand parent Mondelez kicked off the pitch process in April.
Wieden & Kennedy handles the global account, having won it in 2013, but did not pitch for the European business.
O&M London’s first work for the brand is likely to be launched towards the end of the year.
Charlie Rudd, chief executive at O&M London, said: "We couldn’t be more delighted to work with the Halls team, from the very first meeting we connected about the opportunity for a fresh creative outlook for the brand – we can’t wait to get started."