O2 may refer to:
- O2, consumer brands used by the Spanish telecommunications company Telefónica, the others being Movistar, used in Spain and Spanish American countries, and Vivo, used in Brazil. Telefónica acquired the O2 mobile networks in 2005–2006. Telefónica-owned O2 networks operate in the following countries:
- United Kingdom. O2 was a subsidiary of BT before being spun off in 2002.
- Germany. O2 is the largest mobile network after the 2014 acquisition of E-Plus from KPN.
- Spain, Telefónica's home country. O2 operates alongside Movistar and Tuenti.
The O2 brand is licensed to the Czech investment company PPF Group for use in the following countries (Telefónica owned the networks until 2013):
Furthermore, O2 also operated in Ireland until Hutchison Whampoa acquired it in 2015 and discontinued the O2 brand in favour of its own Three brand.