The chosen agency will be charged with 'driving talking' around O2's current products and future product launches.
The move to add to its agency line-up forms part of O2's parent company Telefónica's aim to integrate its comms more between press office, media relations and external messaging functions.
Telefónica director of comms and reputation Nicola Green confirmed that a pitch process was under way.
She said: 'We want an agency that can work with our internal teams, as well as our agencies, to make sure all our comms work fits seamlessly together.
'The successful agency will sit between strategy and consumer, help us communicate good news stories that often become lost and amplify our campaigns.
'The agency will also help drive talking around our products and increase awareness of the services we offer.'
O2 has shortlisted a handful of agencies for the brief, with pitches due to be held in two weeks' time.
An appointment is likely to take place before the end of the month. The brief will see the winning agency work alongside the internal team to man its press office.
Telefónica's agency setup includes Blue Rubicon as its lead agency, which holds a £1m-a-year brief.
John Doe was recently hired to handle a rolling project brief providing consumer support and helping O2 to interact with youth and contemporary popular culture, while Quiller Consultants handles public affairs activities.
The move to reinforce its agency support follows reports that O2, Vodafone and 3 have all spoken out at Everything Everywhere's 4GBritain campaign.
Financial results released in February showed that Telefónica had become the largest telecoms company in Europe. O2 is the second largest mobile telecoms provider in the UK after Everything Everywhere.