The company originated as a collection of worldwide telecommunications companies, known in the later half of the 1990s as BT Wireless, and a global mobile data business known then as Genie Internet, both subsidiaries of British Telecommunications (British Telecom or BT).

O2 has additionally established a joint venture with Tesco Mobile in the United Kingdom, Republic of Ireland and in Slovakia, and the Tchibo Mobilfunk network in Germany.

[10] Viag Interkom was created in 1995 as a joint venture of the German power supply firm VIAG (45%), British BT Group (then called British Telecommunications) (45%) and Norwegian Telenor (10%) in order to compete for official licence to provide services in the rapidly liberalising German (fixed and mobile) telecommunications market at that time.

Telfort was created in March 1997 as a 50:50 joint venture between BT and Nederlandse Spoorwegen (the Dutch national railways operator) and headquartered in Amsterdam.

By the time it became a part of the BT Wireless family of companies in 2001, Genie had mobile portal operations in the UK, Germany, Spain, Italy, the Netherlands, Hong Kong and Japan, and an Internet MVNO operation in UK called Genie Mobile.

After demerging from BT in 2001, the European Genie business became the basis of a central products and services division called 'Products O2' and the Genie Mobile business was rebranded to 'O2 Online' which continues in the UK as a mobile communications service provider tied to the O2 UK network.

During this period, O2 Netherlands was sold to Dutch investment group Greenfield Capital Partners in 2003.

On 31 October 2005, O2 plc agreed to be taken over by Telefónica, a Spanish telecommunications company, with a cash offer of £17.7 billion, or £2 per share.

[15] 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 into the governance of O2, which retained its UK registered public company status with its own board of directors and corporate governance structures and processes.

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

O2 Asia, headquartered in Singapore, operating in Far East, South Asia, Middle East, and Australasian countries, for a short time developed and marketed a range of wirelessly connected PDA and smartphone products branded Xda for both the Asian and European markets, and further such products under the "MWg" brand, short for Mobile & Wireless Group.

[16] In 2015 there were talks for Li Ka-shing, owner of the rival UK network Three, to buy the company.

Along the way, rapid modernisation of the network occurred with the help of funding and expertise of the Dutch and Swiss consortium called TelSource.

On 28 February O2 launched classic services and opened first shop in Slovakia (in Bratislava on Obchodná street).

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

The partnership with Irish rugby went interactive in 2006, when Ireland fans were offered access to daily updates from head coach Eddie O'Sullivan.

[24] The integrated campaign was created by O2's two Irish advertising agencies: McConnells (above-the-line) and Brando (below-the-line).

[26] On 31 May 2005, Telefónica O2 acquired the naming rights for the redeveloped Millennium Dome in London from Anschutz Entertainment Group (AEG).

For short time, the naming rights arrangements with AEG also apply in Germany, with large sport and concert venues known as O2 World in Berlin and Hamburg.