It further shares track with the Lambertseter Line along the 2.5-kilometre (1.6 mi) section from Tøyen to Brynseng.

The line runs through the primary residential areas of Bøler, Østensjø and Søndre Nordstrand.

Proposals for making the line part of the metro arose in the late 1940s and the system was approved in 1956.

The Østensjø Line runs from Tøyen on the Common Tunnel to Mortensrud, a distance of 9.0 kilometres (5.6 mi).

[2] At this point there is also a direct, single-track tunnel which allows non-revenue trains access from the Grorud Line.

[2] After Ensjø the line enters a new tunnel at Malerhaugen, which includes the underground Helsfyr Station.

The line then turns southwards and runs 550 metres (1,800 ft) to Godlia Station, which is located on an elevated section of track.

[12] Operations of the lines are done by Sporveien T-banen on contract with Ruter, the public transport authority in Oslo and Akershus.

However, the two municipalities and their tram companies could not agree on terms to lease the trackage from Etterstad to the city center.

Part of the problem was the lack of a suitable turning loop with free capacity in the city center.

For the operations, Akersbanerne bought four Class A trams and three trailers, which were built by Skabo Jernbanevognfabrikk and delivered in 1926 and 1927.

New negotiations between the tram companies were carried out, but neither party was willing to make sufficient admittances for agreement to be reached.

[17] Bærumsbanen, that had become partially owned by Oslo Sporveier, had a similar suburban tramway to the west of the city center, the Lilleaker and Kolsås Lines, where trams continued along street trams lines until they reached the city center.

[17] Bærumsbanen had a fleet of Class A trams, and these were along with Akersbanerne's units put into use on the combined service.

The first Class B trams were delivered to Oslo Sporveier in 1937, and these put into service on the Bærum–Østensjø Line after the serial delivery started in 1939.

The Kolsås Line was connected to the Common Tunnel from 15 June 1942 and became part of the underground service.

The municipalities of Oslo and Aker merged in 1948, and the ownership of the Østensjø Line was transferred to Bærumsbanen.

An overground system was ruled out because of the increased estimates in traffic and an underground route was instead pursued.

There was a large housing shortage in Oslo and the region was experiencing rapid population growth.

The solution was to build a series of commuter towns in Aker, which would be the basis for the traffic on the metro.

[25] The basics for the metro were established in 1951, with a common segment through Enerhaugen with a terminus at Grønlands torg, to avoid having to cross Akerselva.

Investments were estimated at 221 million Norwegian krone, including rolling stock, for all four lines.

The first part of the extension was built as a tramway; work on the 1.4-kilometre (0.87 mi) section to Bøler commenced in 1957.

This included construction of the first tunnel on the Østensjø Line and resulted in Oppsal Station being moved.

As housing development was gradually forcing its way further south in Nordstrand, the municipal council saw the need for extensions to the line.

The total costs of the line from Helsfyr to Skullerud ended at 39 million Norwegian krone.

The three days were used to remove the balloon loop at Bøler and connect the line to the Common Tunnel.

[31] During the last years of the 1980s the barres and payment stalls were removed and replaced with a proof-of-payment system.

The 2.4-kilometre (1.5 mi) extension is built to allow speeds of 100 kilometres per hour (62 mph), and represents a considerable longer distance between stations than what is normal on the network.

[40] Handling of this much increased traffic will minimum require eight trains per hour along the Østensjø Line.