O2 is currently the second-largest mobile phone network provider in the UK. Trading under the name O2 and owned by the Spanish company Telefonica.
O2 has a long history as it was originally called Cellnet. Cellnet was the second mobile phone network provider to be launched in the UK, with the first being Vodafone.
If you were looking to buy a phone in the late 1980s early 1990s the only two network providers at the time were Cellnet and Vodafone. Cellnet was originally owned by Securicor and British Telecom (BT).
The O2 brand was launched while the network provider was owned by BT, who later sold it to Telefonica in 2006.
O2 offer pay monthly, pay as you go and sim only contracts to its UK customers. It also offers 5G to 21 towns and cities. There are plans to roll out 5G further.