John Doe has been appointed to handle a rolling project brief providing consumer support, which was won without a pitch process.
Agency founder Rana Reeves said that his agency was approached late last year by newly promoted director of comms and reputation Nicola Green, who works for O2's parent company Telefonica UK.
He added that the brief would focus on 'looking at how O2 can continue to interact with youth and contemporary popular culture'.
Green, who was last month promoted from head of PR at O2, said it had called on John Doe to 'bring a sense of cool again - we are missing that', explaining that it was the agency's 'cultural network' that attracted her.
'It's about how we can relate to our audiences, what they are and what they do,' she added. 'What we're hoping to do, we used to do a lot of ten years ago, and it helped us create the O2 brand. As it's our birthday it's an opportunity for us to look at what our core values are.'
Green approached Reeves after the two had worked together ten years ago, when he helped to launch the O2 brand while he was at Jackie Cooper PR.
O2 continues to use Blue Rubicon for strategic comms, and last year brought in Hope&Glory to run a pre-Christmas social media campaign.
Telefonica remains on the hunt for an agency to handle PR and public affairs activities for its new global business Telefonica Digital.
The brand recently admitted that it inadvertently handed over subscribers' phone numbers to sites that offered age-restricted material and premium-rate billing, without users' knowledge.
O2 is the second largest mobile telecoms provider in the UK after Everything Everywhere. It was acquired by Telefonica in 2005.