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 (Ireland) also run the network operations for Three Ireland"). The Network team outsource was limited to the Field Operations team along with some other support functions. The Network rollout team & Transmission team remained in-house to facilitate future network changes. It was more cost effective to retain these divisions in-house. In 2006 O2 chose Irish Autism Action as their charity of choice. The three-year deal involved sponsorship, creating awareness, using O2 staff to assist the charity and using technology developed by O2 to assist children affected by autism. In 2010, the company's charity of choice became Headstrong, a charity providing mental