O2 (United Kingdom)
 O2 UK redirects here. For other uses see O2 (disambiguation).Telefónica UK Limited (trading as O2 – stylised as O2) is a telecommunications internet and financial services provider in the United Kingdom owned by Telefónica forming part of its Telefónica Europe division. It is the second-largest mobile telecommunications provider in the United Kingdom (after EE) and is headquartered in Slough.O2 was formed in 1985 as Cellnet a 60:40 joint venture between BT Group and Securicor.