O2 UK (legally incorporated as Telefonica UK Ltd.) is a British telecommunications services provider, owned by Telefónica, headquartered in Slough, England. O2 is the second-largest mobile network operator in the United Kingdom, with 26.4 million subscribers as of February 2020, after EE, and followed by Vodafone and Three.
The company was formed in 1985 as Cellnet, at first a joint venture between BT Group and Securicor, later wholly owned by BT. Cellnet was one of the first two mobile networks in the UK, alongside Vodafone. BT demerged its mobile division in 2002 as mmO2 plc, which introduced the „O2“ brand for the businesses. In 2006 the company was purchased by Telefónica, who retained the O2 brand and the company’s UK headquarters. Telefónica and Liberty Global reached an agreement in May 2020 to merge the company with Virgin Media.
O2 was formed on 7 January 1985 as Cellnet, a 60:40 joint venture between BT Group and Securicor. In 1999, BT acquired Securicor’s share of Cellnet and the company was later rebranded as BT Cellnet. In June 2000, BT Cellnet launched the world’s first commercial General Packet Radio Service (GPRS) service. The company, together with BT Group’s mobile telecommunications businesses in Germany, Ireland and the Netherlands, was part of the BT Wireless division. This was spun off from the BT Group in 2002 to form a new holding company, mmO2 plc, which introduced the new „O2“ brand for the businesses. In 2005, mmO2 plc was renamed O2 plc.