The first section of the line was built by Akersbanerne as part of the Oslo Tramway and connected to the Vålerenga Line at Etterstad. The first section, to Bryn, opened on 18 December 1923, and was extended to Oppsal in January 1926. Services were variously provided by Kristiania Sporveisselskab, Bærumsbanen and Oslo Sporveier. Proposals for making the line part of the metro arose in the late 1940s and the system was approved in 1956. This materialised in an extension of the line to Bøler on 20 July 1958. The Østensjø Line became the third line of the metro on 26 November 1967, the same day the line was extended to Skullerud. The last extension, to Mortensrud, commenced services on 24 November 1997. There are proposals to extend the line further south to Bjørndal and Gjersrud.