Telefónica, S.A. is a Spanish multinational telecommunications company headquartered in Madrid, Spain. It is one of the largest telephone operators and mobile network providers in the world. It provides fixed and mobile telephony, broadband, and subscription television, operating in Europe and the Americas.

Telefonica UK Limited, trading as O2 UK (stylised as O2), is a British telecommunications services provider, headquartered in Reading, England which operates under the O2 brand. It is owned by VMED O2 UK Limited, a 50:50 joint venture between Telefónica and Liberty Global. O2 is the UK's largest mobile network operator, with 23.8 million subscribers as of May 2023.