O2 is one of the 4 UK mobile phone operators, currently owned by Telephonica, a Spanish company. O2 is the second largest of the UK operators with 20.9% of the market share. It is one of the two original mobile operators in the UK. The original UK market was established with Vodafone and Cellnet (that later became O2). Cellnet was originally owned by BT but later split off and sold to the present owners. Cellnet established itself as an analogue mobile phone service prior to developing 2G, 3G and 4G digital services.

O2 have a 99% population coverage for their 2G 3G / 4G networks. Their download speeds average 14.61 with a latency of 42ms making it the worst of the 4 mobile network operators.

The frequencies they use are :

800MHz	Band 20	4G
900MHz	Band 8	2G + 3G
1800MHz	Band 3	2G + 3G
2100MHz	Band 1	3G + 4G
2300MHz	Band 7	4G
3400MHz	Band 42	5G

Over the years, due to various spectrum auctions and OfCom allocations, O2 have accumulated a lot of frequency over a number of spectrum areas. Perhaps because of this O2 did not purchase any of the 2.6MHz frequency within which to run their 4G network. In the latest 5G auction, maybe because of the lack within 2.6MHz, purchased some spectrum within 2.3MHz to expand their 4G capacity plus some of the 3.4MHz bandwidth which they plan to use for 4G.

O2 will be the last to run a 5G network, planning to start in 4 UK cities (London, Edinburgh, Cardiff and Belfast) in Q3 2019. They have be involved with many trial of their equipment. O2 have been focusing on delivering 4G to nearly 100% of the UK population with only the difficult homes omitted. Their strategy with 5G is likely to be the same, with the assumption that with the amount of 800/900MHz bandwidth would have the coverage capacity, certainly with any new 700MHz licenses. O2 also support a network of O2 WiFi hotspots which can be combined well with 5G in providing 5G capacity.

O2 have a large online and retail presence selling monile phones, and other online devices, that can be attached to their network.