The agency, which won the account after an eight-way pitch, will be responsible for rebuilding all of O2's digital footprint - including its website o2.co.uk - as well as its tablet and mobile sites. It has been briefed to create a new "digital experience" that has personalisation at its core.
The appointment does not affect VCCP, which continues to handle O2's digital marketing and advertising.
Lance Haswell, the head of online at O2, said: "We are committed to a cutting-edge approach to our digital offering, and are delighted that Tribal DDB shares our ambition for a future-proof, best-in-class experience."
Tom Roberts, the managing director of Tribal DDB, added: "Tribal DDB will ensure the brand stays ahead of its competitors and delivers the world-class digital experience its customers have grown to expect."