O2 (UK) they can do it too" in reference to Vodafone and EE, O2's biggest competitors and the UK's top three mobile-telecoms companies. 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
O2 (UK) billion (£2 per share) in cash. It went through finally in 2006. According to the merger announcement, O2 retained its name and continued to be based in the United Kingdom, keeping both the brand and the management team. The merger became unconditional on 23 January 2006. Following the acquisition of O2, Telefónica undertook a corporate organisational change that saw the merging of its fixed and mobile businesses in Spain, and the transfer of Telefónica's non-Spanish European telecommunications properties into the O2 brand. Thus, the Český Telecom and Eurotel operations in the Czech Republic as well as the Telefónica Deutschland business