O2 (UK) O2 (UK) Telefónica UK Limited (trading as O2 – stylised as O) is a telecommunications services provider in the United Kingdom, owned by the Spanish multinational Telefónica, and is headquartered in Slough. With 25 million subscribers (as of December 2017), O2 is the second-largest mobile network operator in the United Kingdom after EE, with Vodafone in third place. O2 was formed in 1985 as Cellnet, a 60:40 joint venture between BT Group and Securicor. In 1999, BT Group acquired Securicor's share of Cellnet and the company was later rebranded as BT Cellnet. In June 2000, BT Cellnet launched the world's
LSWR O2 class to gain acceleration on intensive timetables. Adams settled upon the 0-4-4T wheel arrangement to provide the basis of what was to become the O2 Class. The second of William Adam's 0-4-4 designs, the O2 Class was a development of his previous T1 class of 1888. The brief behind the design was to create a locomotive capable of mixed-traffic operations, a characteristic dictated by the relatively small wheel diameter and smaller cylinders, effectively to replace the obsolete Beattie Well Tank. As a result, a compact locomotive with high route availability was produced, a factor that would be essential during the later