In May, the agency won the European advertising account for sister Mondelez brand Halls following a competitive pitch.
Kelly Rafferty, marketing director, gum and candy EEMEA, at Mondelez: said "The new assignment reflects our ambition to grow our candy business in the UK and modernize the Trebor brand in order to engage the next generation of mint lovers."
Both Halls and Trebor brands were previously handled by Wieden & Kennedy.
Wiedens was responsible for creating the first TV ad for Trebor in a decade – the 2015 spot showed how choosing the brands soft and extra strong mint was a matter of family honour.
Charlie Rudd, the chief executive at O&M London, added: "Trebor is an iconic British brand which deserves our very best work and we look forward to working with the Mondelez team to makes sure we deliver that."