O2 (previously known as Cellnet) uses a GSM frequency of 900 MHz, although it also uses 1800 MHz frequencies to increase capacity. Providing 99% coverage of the UK, 02 achieved a national call success rate of 99.2% in the period between April to September 2003 - higher than any other network at that time.
The map below shows the network coverage for O2 across the UK. White areas show regions of little or no coverage.