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) 2016, the European Commission officially blocked the tie-up of O2 and Three, arguing that the merger would reduce consumer choice and lead to a higher cost of services. In September 2016, Telefónica appointed a number of investment banks to sell the business to investors, ahead of a stock market flotation. On 6 December 2018, a major disruption to the O2 network, caused by faulty software, left up to 32 million users (including those on MVNOs) without access to data services (on both 3G and 4G) for up to 24 hours. During the outage, some voice and text services suffered from