The review, which is thought to be statutory, is expected to start in April and will be run from Madrid. It could be expanded to include markets outside Europe.
O2’s UK media is held by ZenithOptimedia, which also works with Telefonica in Germany. Telefonica’s other European media business is held by a number of agencies.
O2 spent £49 million in the UK in the year to September 2012, according to figures from Nielsen. It was the 18th-biggest-spending advertiser in the UK during that period.
ZenithOptimedia has held the UK business since 2004 and retained it in 2010.
Telefonica has been attempting to return to financial stability by cutting its net debts, which stood at €4.46 billion at the end of the third quarter in 2013.
Since its last media review, the telecoms giant has sold businesses including O2 in Ireland and Telefonica Czech Republic. ZenithOptimedia ran the media for both.
The company is expanding in Germany and agreed to buy E-Plus from the Dutch group KPN in July 2013 for $10.7 billion. It plans to combine E-Plus, which has more than 20 million customers, with O2 in Germany.
It is possible that Telefonica will look to consolidate all of its operations into one media agency.
An O2 spokesman told Campaign: "We do not comment on rumour or speculation. We regularly review our relationship with agency partners throughout the business but have no news to share at this time."
No-one at ZenithOptimedia would comment.
Separately, it has emerged that Vodafone is reviewing its £600 million global media planning and buying account, held by Omnicom’s OMD.