O2 (UK)

Telefonica UK Limited
Trade name	O2 (2002–present); BT Cellnet (1999–2002); Cellnet (1985–1999);
Formerly	Telecom Securicor Cellular Radio Limited (1983–2000); BT Cellnet Limited (2000–2002); O2 (UK) Limited (2002–2008); Telefónica O2 UK Limited (2008–2011); Telefonica O2 UK Limited (2011);
Type	Subsidiary
Industry	Telecommunications
Founded	28 July 1983
Founder	John Carrington
Headquarters	Reading, England
Area served	United Kingdom
Key people	Lutz Schüler (CEO)
Revenue	£6.510 billion (2017)
Parent	Virgin Media O2
Subsidiaries	giffgaff; Tesco Mobile (50%);
Website	o2.co.uk

Telefonica UK Limited, trading as O2 UK (stylised as O₂), is a British telecommunications services provider. It is the second largest mobile network in the United Kingdom behind VodafoneThree, with approximately 22.4 million subscribers as of December 2025. The name derives from the oxygen molecule.

In 2021, O2 UK formed a subsidiary called Virgin Media O2, a 50:50 joint venture between Telefónica and Liberty Global, through the merger of their respective O2 UK and Virgin Media businesses.

The network was launched in 1985 as Cellnet, a joint venture between British Telecom (60%) and Securicor (40%), and later rebranded BT Cellnet following BT's acquisition of Securicor's share. Cellnet was one of the two original cellular network operators in the UK, alongside Vodafone. In 2001, BT spun off its BT Wireless division as mmO2 plc (later O2 plc), with the UK network adopting the O2 brand in 2002. O2 plc was acquired by Spanish telecommunications firm Telefónica in 2006.