Agencies have already submitted creds and a shortlist including the incumbent has been drawn up by the UK's second-biggest mobile operator.
An O2 spokesperson said: "We are very happy with our current support, which has been in place for four years, but together with our procurement team we are keen to see what else is on offer to ensure we remain fresh and industry-leading."
The Telefonica-owned operator has worked with Blue Rubicon since 2009, when it consolidated its PR duties with the agency without a pitch.
The account was estimated to be worth a seven-figure sum at the time. However, O2 would not comment on the size of the current brief or whether the work could be split between different agencies.
It has employed Hope&Glory in the past year for a consumer-focused brief aimed at increasing "talkability" around the brand and works with John Doe on a rolling project basis.
It is understood that neither agency is affected by the review.
In the battle for 4G customers, O2 introduced a multimillion-pound advertising campaign and new brand strategy in July featuring the slogan "Be more dog".
O2 and rival Vodafone began their 4G roll-outs at the end of August, almost a year after EE, which has since established itself as the leading 4G provider in the UK.
In October O2's in-house team was named PRWeek's In-House Team of the Year.