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 (UK) UK. The deal was subject to regulatory approval. The European Commission decided in December 2015 not to refer Hutchison's takeover of Telefónica's O2 business in the UK to the country's Competition and Markets Authority, which had asked to be allowed to investigate the planned acquisition, arguing that as the British competition regulator, it – and not the Commission – should have the right to rule on the 'transaction', which it argued 'threatens to affect significantly competition in the UK retail mobile and wholesale mobile markets', claiming that its investigation of the takeover would 'avoid duplication and fragmentation'. On 11 May