The Common Tunnel (Norwegian: Fellestunnelen), sometimes called the Common Line (Fellesstrekningen), is a 7.3-kilometer (4.5 mi) long tunnel of the Oslo Metro which runs through the city center of Oslo, Norway. The name derives from the fact that all five lines of the metro use the tunnel, which runs from Majorstuen to Tøyen. The section has six stations, including the four busiest on the metro.

The tunnel was first built as two separate tunnels which were later connected. The Holmenkolbanen company opened the western section of the tunnel from Majorstuen via Valkyrie plass to Nationaltheatret in 1928. In 1966, the Oslo Metro opened, including the tunnel from Tøyen via Grønland to Jernbanetorget. In 1977, the eastern end was extended to Sentrum, but the extension was closed in 1983 because of water leakages. In 1987, the Sentrum station reopened as Stortinget, and became the terminus of both the western and eastern lines. By 1993, the western end had been upgraded to metro standard, Valkyrie plass was closed, and the first trains ran through the whole tunnel.

The section from Tøyen to Brynseng, although not completely located within the tunnel, is considered part of the Common Line. At Majorstuen, the line splits into three; the Røa Line, the Holmenkollen Line and the Sognsvann Line. At Tøyen, the line splits into the Lambertseter Line and the Grorud Line. The tunnel is the bottleneck of the metro, allowing 24 trains per hour in each direction west of Stortinget, and 28 east of Stortinget.

The Common Tunnel is a 7.3-kilometer (4.5 mi) long double track rapid transit line running east from Majorstuen to Tøyen and onwards to Brynseng. The section from Tøyen to Brynseng is partially above ground and is only sometimes considered part of the tunnel and the Common Line. At Tøyen, the Grorud Line branches and continues past Carl Berners plass as part of the same tunnel. From Carl Berners plass to Ensjø, there is a single-track branch, which allows trains access to the other eastern lines without having to change direction. Three stations serve the city center: Nationaltheatret, Stortinget and Jernbanetorget, while Majorstuen, Tøyen and Grønland serve mixed residential and commercial areas.

All five lines of the Oslo Metro, numbered 1 to 5, run through the Common Tunnel. Most services have a 15-minute headway, which is increased to 30 minutes in the late evening and early on Sundays. Two lines have more frequent service. Line 2, which runs on the Furuset Line, has its headway reduced to 7.5 minutes and runs the entire tunnel. Line 3 on the Østensjø Line also has extra trains that terminate at Stortinget giving a 7.5 minute frequency on the east side of the tunnel. Line 5 trains run once around the Ring Line before continuing to their final destinations and therefore make two passes through the tunnel on a complete journey. Travel time from Majorstuen to Tøyen takes eight minutes. The tunnel is the bottleneck of the metro, with a capacity for 24 trains per hour in each direction west of Stortinget, and 28 east of Stortinget.

Transfer to the Oslo Tramway is available at four stations, Majorstuen, Nationaltheatret, Stortinget and Jernbanetorget. Transfer to the mainline railway is available at Nationaltheatret to the railway station with the same name, and at Jernbanetorget to Oslo Central Station (Oslo S). Nationaltheatret offers shorter transfer distance, but only serves trains along the Drammen Line. Interchange with buses is possible at all stations except Grønland. Majorstuen acts as an interchange between the western lines, while transfer between the eastern lines can be done at Tøyen. The public transport authority Ruter recommends that interchange between lines in the same direction should be made at Grønland.

In 1898, the Holmenkollen Line was opened as a light rail between Majorstuen to Holmenkollen. The terminus at Majorstuen was an inconvenience for passengers as most people were bound for the city center and had to change to the street trams. For the operator Holmenkolbanen, there were two ways to solve the issue. They could sign trackage rights with Kristiania Elektriske Sporvei to use the Briskeby Line to reach the city center. Alternatively, a tunnel could be built to an underground station in the city center. Applications for a tunnel were sent to the municipality in 1901. However, disagreements as to whether the tunnel was to be considered a railway or tramway halted progress; the city considered the line a tramway while the company considered it to be a railway.

A new application was submitted in November 1907, which proposed a single track line to the intersection of Karl Johans gate and Fredriks gate, and was estimated to cost 1.5 million Norwegian krone (NOK). The plans drew criticism from city officials, who stated that the line would take passengers away from the existing tram line and that it would stimulate migration to the areas along the Holmenkollen Line in the neighboring municipality of Aker, thus reducing the tax income for the city. In 1909, Oslo City Council stated in a letter to the government that they were opposed to the tunnel. On 9 June 1911, the city council had nonetheless changed its mind, and voted to accept the tunnel if the terminus was moved to the intersection of Karl Johans gate and Ruseløkkveien, but this was rejected by the company. On 27 May 1909, Aker Municipal Council voted in favor of the tunnel, and on 9 June 1911 the proposal was passed by the Parliament of Norway. Both municipalities wanted the redemption right, which would allow them to purchase the railway at a later date, but this was instead given to the state. On 15 December 1911, Holmenkolbanen received a concession through a royal decree, which would last for 60 years from the date of opening.