O2 (UK) O2 (UK) Telefónica UK Limited (trading as O2 – stylised as O) is a telecommunications services provider in the United Kingdom, owned by the Spanish multinational Telefónica, and is headquartered in Slough. With 25 million subscribers (as of December 2017), O2 is the second-largest mobile network operator in the United Kingdom after EE, with Vodafone in third place. O2 was formed in 1985 as Cellnet, a 60:40 joint venture between BT Group and Securicor. In 1999, BT Group acquired Securicor's share of Cellnet and the company was later rebranded as BT Cellnet. In June 2000, BT Cellnet launched the world's
O2 (Ireland) their bills on time in August 2008. The Advertising Standards Authority upheld complaints concerning advertisements by O2. In August 2010, O2 was warned by the telecoms watchdog, ComReg, that it could not move customers to online billing without their explicit agreement. In March 2011, O2 pleaded guilty to a breach of the Data Protection Act at the Dublin District Court. In March 2011, it was revealed that Denis O'Brien made payments to the Minister for Communications, Michael Lowry, to aid Esat Digifone's licence bid. O2 (Ireland) Telefónica Ireland was a broadband and telecommunications provider in Ireland that traded under the