The Telefonica-owned brand has handed the business, worth an estimated £1m, to Blue Rubicon without a pitch. The decision comes into effect on 1 January.
Talk PR has handled small business while Skywrite has overseen business and technology for the past two years.
O2 has worked with Blue Rubicon on the launch of its O2 Money product in recent months. O2 PR manager Ken Leitch told PRWeek the decision was based on the company's 'changing needs as a brand' and the 'changing PR landscape'.
Skywrite declined to comment on the development.
Talk PR board director Sara Jensen, meanwhile, said her agency continued to work for O2. 'We have not severed our relationship,' she said. 'We are working on projects that do not involve O2's PR department.'
O2 stole a march on its rivals when it secured the exclusive iPhone contract in 2007. That exclusivity will expire shortly, with Orange and Vodafone due to start offering iPhone service contracts.
In addition, O2 has faced sustained criticism, particularly online, over the quality of its network and signal coverage.
Leitch said: 'What I want to be clear about is that this has been a long process and something we have been working on for a number of months. As such, it is not a knee-jerk reaction.'
O2 recently hired Quiller Consultants to handle public affairs. The agency took the account from Bell Pottinger Public Affairs following a competitive pitch. Mission Media will continue to handle O2's music sponsorship PR.