Gainford joins the agency from DDB where he oversaw the £36m Virgin Media business before it was handed to Bartle Bogle Hegarty in December.
At Lida, Gainford will oversee the £24m O2 direct marketing account, which the agency won in November from incumbent Archibald Ingall Stretton.
Gainford will be responsible for O2's overall proposition of "fresh thinking new possibilities", overseeing the 40-stronghold team. He will report into Mel Cruickshank, chief executive.
Prior to working at DDB, Gainford spent two years at Elvis and Dare, working on accounts including Virgin Trains, Virgin Holidays and T-Mobile.
Cruickshank said: "We feel Nathan's experience in brand and integrated comms, alongside his solid CRM credentials, makes him a very strong Managing Partner on O2.
"He will complement our senior agency team perfectly."