O2 is the largest mobile network operator in Germany by subscribers following the 2014 acquisition of the E-Plus network from the Dutch company KPN. It has been a subsidiary of Telefónica, a Spanish telecom multinational, since 2005. Its competitors are Telekom Deutschland and Vodafone.

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In 2001, BT Wireless demerged from BT and took the name mmO2 plc. While mmO2 plc was created in 2001, the O2 consumer brand was introduced on 19 June 2002 in Germany. Similar rebrands took place in the United Kingdom, Ireland and the Netherlands (where O2 lasted for only a year before reverting to Telfort).

Spanish telecommunications company Telefónica acquired O2 in the United Kingdom, Ireland and Germany in early 2006.

Telefónica acquired the E-Plus network from the Dutch telecommunications company KPN in 2014. The E-Plus brand was discontinued in 2016. The merger made O2 Germany's largest telecom operator, ahead of Telekom.