Telefonica UK Limited, trading as O2 (stylised as O2), is a British telecommunications services provider, headquartered in Reading, England which operates under the O2 brand.

[15] Cellnet was established in 1985 as a joint subsidiary of BTCR, British Telecom Cellular Radio, providing the engineering knowledge, and TSCR, Telecom Securicor Cellular Radio Limited, providing the financial investment, resulting in a 60:40 joint venture between British Telecommunications and Securicor.

[16] After months of rumours and speculation, Peter Bonfield publicly announced on 27 July 1999 that BT had agreed to buy Securicor's 40 per cent share of Cellnet for £3.15 billion.

[20] According to the merger announcement, O2 retained its name and continued to be based in the United Kingdom, keeping both the brand and the management team.

Thus, the Český Telecom and Eurotel operations in the Czech Republic as well as the Telefónica Deutschland business in Germany were brought under the control of O2, which retained its UK-registered public company status with its own board of directors and corporate structures and processes.

Telefónica chose to keep its existing mobile phone operations in the rest of the world under the brand Movistar.

Future plans included manufacturing Near Field Communication (NFC) technology in mobile phones in the United Kingdom.

[21] O2 and Vodafone signed a deal in June 2012 which will see the two companies 'pool' their network technology, creating a single national grid of 18,500 transmitter sites.

[24] On 23 January 2015, Li Ka-shing, owner of Hutchison Whampoa and the Three network (which operates in the UK) entered talks to buy O2 for up to £10.25bn ($15.4bn).

However, the move faced scrutiny from competition regulators as it would reduce the number of major operators in the UK, when combined with a potential purchase of EE by BT, from four to three.

[29] On 11 May 2016, the European Commission officially blocked the tie-up of O2 and Three, arguing that the merger would reduce consumer choice and lead to a higher cost of services.

[30] In September 2016, Telefónica appointed a number of investment banks to sell the business to investors, ahead of a stock market flotation.

[31] On 7 May 2020, it was announced that Telefónica had agreed to merge Telefonica UK with Liberty Global subsidiary Virgin Media, subject to approval, into a 50/50 joint venture.

[34] In July 2012, O2 had to apologise to almost 8 million customers after a network switching subsystem failure led to a 24-hour blackout of the service across the UK and Ireland.

On 6 December 2018, a major disruption to the O2 network, caused by faulty software, left up to 32 million users (including those on MVNOs) without access to data services (on both 3G and 4G) for up to 24 hours.

[44] The 4G LTE service became available to customers in London, Leeds and Bradford on 29 August 2013, and expanded to a further ten cities by the end of the year.

O2 announced in June 2011 a fibre optic broadband service designed to compete with the BT Infinity product, using FTTC technology.

[50] BSkyB agreed on 1 March 2013 to buy the fixed telephone line and broadband business of Telefonica UK, trading under the O2 and Be brands.

[53] In 2009, O2 was in discussions with large retailers, such as Tesco and W. H. Smith, for the deployment of the necessary electronic point of sale units, and with handset manufacturers, such as Apple and Samsung, to enable NFC technology on all future devices.

[54] On 23 February 2011, O2 announced it would launch a "second phase" for O2 Money, by discontinuing its branded cash cards in favour of a "mobile wallet" application for Android and iOS devices.

The BT Cellnet consumer brand was renamed O2 – the chemical symbol for an oxygen molecule – as were all the group's other businesses (other than Manx Telecom).

In February 2017, in a continuation of its agreement with AEG, the developers of the site, O2 announced that it had renewed its naming rights for a further ten years till 2027.

O2 attained the standard after saving 47,000 tonnes of carbon over the previous three years through its energy efficiency measures, including a £1.4 million distribution of smart metering technology across the company's cell sites, offices and retail stores, and upgrades to more energy efficient systems across its mobile phone network.

[67] In October 2017, an inquiry was launched in the UK concerning overcharging customers for handset sales integrated with the data and call charges.