The Oslo Metro (Norwegian: Oslo T-bane or Oslo Tunnelbane or simply T-banen) is the rapid transit system of Oslo, Norway, operated by Sporveien T-banen on contract from the transit authority Ruter. The network consists of five lines that all run through the city centre, with a total length of 85 kilometres (53 mi), serving 101 stations of which 17 are underground or indoors. In addition to serving 14 out of the 15 boroughs of Oslo, two lines run to Kolsås and Østerås, in the neighbouring municipality of Bærum. In 2016, the system had an annual ridership of 118 million.

The first rapid transit line, the Holmenkollen Line, opened in 1898, with the branch Røa Line opening in 1912. It became the first Nordic underground rapid transit system in 1928, when the underground line to Nationaltheatret was opened. After 1993 trains ran under the city between the eastern and western networks in the Common Tunnel, followed by the 2006 opening of the Ring Line. All the trains are operated with MX3000 stock. These replaced the older T1000 stock between 2006 and 2010.

Rail transport in Oslo started in 1854, with the opening of Hoved Line to Eidsvoll, through Groruddalen. In 1872, Drammen Line, going through Oslo West, and in 1879, Østfold Line going through Nordstrand opened, offering a limited rail service to those parts of the city. By 1875, Kristiania Sporveisselskab (KSS) opened the first horsecar trams. In 1894 electric trams were in service by Kristiania Elektriske Sporvei (KES).

The first suburban tram line was the Holmenkollen Line that was opened by Holmenkolbanen in 1898; like all the later suburban tram line these were electric trams with a grade-separated right-of-way and proper stations instead of tram stops, making it the first rapid transit in Oslo. Unlike the other suburban tram lines that were built later, the Holmenkollen Line was not extended into the city as a streetcar—instead passengers had to change at Majorstuen to the streetcars, though the system did not take into use wider suburban stock (3.1 metres (10 ft 2 in)) until 1909. A branch line was opened in 1912, to Smestad, and in 1916 the Holmenkollen Line was extended to Tryvann, with the last part from Frognerseteren single track and used for freight, and removed in 1939.

In 1912, the construction of the first underground railway in the Nordic Countries started, when A/S Holmenkolbanen started construction of an extension of their line from Majorstuen to Nationaltheatret. The 2.0 kilometres (1.2 mi) line was opened in 1928, with one intermediate station at Valkyrie Plass, giving the two suburban lines access to the central business district of Oslo. In retrospect, it is seen as an early premetro example. It was the second underground railway to be opened in the Nordic countries after Boulevardtunnlen in Copenhagen which opened in July 1918.

The success of the suburban lines tempted KES to extend their streetcar service west from Skøyen as a suburban line; the Lilleaker Line opened to Lilleaker in 1919, to Avløs in 1924 and to Kolsås in 1930. A new section from Jar to Sørbyhaugen opened in 1942, connecting the line from Jar to Kolsås to Nationaltheatret, and making it a rapid transit and the replacement of stock with wide suburban standard. This service remained part of the municipal Oslo Sporveier, that had bought all the streetcar companies in 1924.

Compensation for large amounts of damage to houses along the route during construction, along with higher construction costs than calculated was a heavy burden on the company, and in 1934, the municipality of Aker took over the common stock, though the preferred stock remained listed on the Oslo Stock Exchange until 1975, as Oslo Sporveier gradually took over the operation of the western suburban lines. Akersbanerne opened the connecting Sognsvann Line in 1934.

The first idea to launch a citywide rapid transit was launched in 1912 with the construction of the Ekeberg Line; constructed with the same width profile as the Holmenkollen Line, the plan was to build a tunnel under the city center and run through trains, but large cost expenditures on the first section of the Common Tunnel ceased the plans. As part of the rebuilding after World War II a planning office for a T-bane was established in 1949, with the first plans launched in 1951; in 1954, the city council decided to build the T-bane network in Eastern Oslo with four branches. The system would feature improvements over the suburban lines in having a third rail power supply, cab signaling with Automatic Train Protection, stations long enough for six-car trains and level crossings replaced by bridges and underpasses—specifications christened metro standard.