Oslo is a municipality, as well as the capital and most populous city in Norway. As a municipality (formannskapsdistrikt), it was established on 1 January 1838. Founded around 1048 by King Harald III of Norway, the city was largely destroyed by fire in 1624. The city was moved under the reign of Denmark–Norway's King Christian IV. It was rebuilt closer to Akershus Castle, as Christiania (also spelt Kristiania in late 1800's). In 1925, the city reclaimed its original Norwegian name, Oslo. The diocese of Oslo is one of the five original dioceses in Norway, which originated around the year 1070.

Oslo is the cultural, scientific, economic and governmental centre of Norway. The city is also a hub of Norwegian trade, banking, industry and shipping. It is an important centre for maritime industries and maritime trade in Europe. The city is home to many companies within the maritime sector, some of which are amongst the world's largest shipping companies, shipbrokers and maritime insurance brokers. Oslo is a pilot city of the Council of Europe and the European

Commission intercultural cities programme.
Oslo is considered a global city and ranked "Beta World City" in studies performed by the Globalization and World Cities Study Group and Network in 2008. For several years, Oslo has been listed as one of the most expensive cities in the world along with such other global cities, as Zurich, Geneva, Copenhagen, Paris, and Tokyo. In 2009, however, Oslo regained its status as the world's most expensive city. A survey conducted by ECA International in 2011 placed Oslo 2nd after Tokyo.

As of 2010, the metropolitan area of Oslo has a population of 1,442,318 of whom 912,046 live in the contiguous conurbation. The population currently increases at a record rate of 1.64% annually, making it the fastest growing city in Europe. This growth stems from immigration, as the Norwegian population in the city is decreasing. The immigrant share of the population in the city proper now counts more than 25% of the city's total.

The population of the municipality of Oslo is 605,318 as of 1 Apr. 2011. The urban area extends beyond the boundaries of the municipality into the surrounding county of Akershus, (municipalities of Bærum, Asker, Røyken, Lørenskog, Skedsmo, Gjerdrum, Sørum, Oppegård) its agglomeration total 912,046 inhabitants. The metropolitan area of Oslo, also referred to as the Greater Oslo Region (Norwegian: Stor-Osloregionen), has a land area of 8,900 km2 (3,400 sq mi) with a population of 1,422,442 as of 1 Apr. 2010. The Inner Oslo Fjord Region, or the Capital Region made up by the 5 counties of Oslo, Akershus, Buskerud, Vestfold (west bank of the Oslo fjord) and Østfold (east bank) has a population of 1,908,231 people (01.10.2010). The city centre is situated at the end of the Oslofjord, from which point the city sprawls out in three distinct "corridors" from its centre; inland north-eastwards and southwards along both sides of the fjord giving the city area more or less the shape of a reclining "Y" when seen from the north. To the north and east, wide forested hills (Marka) rise above the city giving the location the shape of a giant amphitheatre. The urban municipality (bykommune) of Oslo and county of Oslo (fylke) are two parts of the same entity, making Oslo the only city in Norway where two administrative levels are integrated. Of Oslo's total area, 115 km2 (44 sq mi) is built-up and 7 km2 (2.7 sq mi) is agricultural. The open areas within the built-up zone amount to 22 km2 (8.5 sq mi).

The city of Oslo was established as a municipality on 3 January 1838 (see formannskapsdistrikt). It was separated from the county of Akershus to become a county of its own in 1842. The rural municipality of Aker was merged with Oslo on 1 January 1948 (and simultaneously transferred from Akershus county to Oslo county). Furthermore, Oslo shares several important functions with Akershus county. The neighbouring industrial commune of Aker was incorporated into Oslo in 1948.

Oslo occupies an arc of land at the northernmost end of the Oslofjord. The fjord, which is nearly bisected by the Nesodden peninsula opposite Oslo, lies to the south; in all other directions Oslo is surrounded by green hills and mountains. There are 40 islands within the city limits, the largest being Malmøya (0.56 km2/0.22 sq mi), and scores more around the Oslofjord. Oslo has 343 lakes, the largest being Maridalsvannet (3.91 km2/1.51 sq mi).

This is also a main source of drinking water for large parts of Oslo.
Although Eastern Norway has a number of rivers, none of these flow into the ocean at Oslo. Instead Oslo has two smaller rivers: Akerselva (draining Maridalsvannet, which flows into the fjord in Bjørvika), and Alna. The waterfalls in Akerselva gave power to the first modern industry of Norway in the 1840, and later in the century, the river became the symbol of the stable and consistent economic and social divide of the city into an East End and a West End; the labourers' neighbourhoods lie on both sides of the river, and the divide in reality follows Uelands street a bit further west. River Alna flows through Groruddalen, Oslo's major suburb and industrial area. The highest point is Kirkeberget, at 629 metres (2,064 ft). Although the city's population is small compared to most European capitals, it occupies an unusually large land area, of which two thirds are protected areas of forests, hills and lakes. Its boundaries encompass many parks and open areas, giving it an airy and green appearance.

Parks and recreation areas
Oslo has a large number of parks and green areas within the city core, as well as outside it.

Frogner Park is a large park located a few minutes walk away from the city centre. This is the biggest and most reputed park in Norway with a large collection of sculptures of Gustav Vigeland

Bygdøy is a huge and green area, commonly called the Museum Peninsula of Oslo. The beautiful location, surrounded by the sea, makes it the most expensive Norwegian district.

St. Hanshaugen Park is an old public park on a high hill in central Oslo. 'St. Hanshaugen' is also the name of the surrounding neighbourhood as well as the larger administrative district (borough) that includes major parts of central Oslo.

Tøyen Park stretches out behind the Munch Museum, and is a vast, grassy expanse. In the north, there is a viewing point known as Ola Narr. The Tøyen area also includes the Botanical Garden and Museum belonging to the University of Oslo.

Oslo (with neighbouring Sandvika-Asker) is built in a horseshoe shape on the shores of the Oslofjord and limited in most directions by hills and forests. As a result, any point within the city is relatively close to the forest. There are two major forests bordering the city: Østmarka (literally "Eastern Forest", on the eastern perimeter of the city), and the very large Nordmarka (literally "Northern Forest", stretching from the northern perimeter of the city deep into the hinterland).true

The municipality operates eight public swimming pools. Tøyenbadet is the largest indoor swimming facility in Oslo and one of the few pools in Norway offering a 50-metre main pool. The outdoor pool Frognerbadet also has the 50-metre range.

Oslo's cityscape is being redeveloped as a modern city with various access-points, an extensive metro-system with a new financial district and a cultural city. In 2008, an exhibition was held in London presenting the award-winning Oslo Opera House, the urban regeneration scheme of Oslo's seafront, Munch/Stenersen and the new Deichman Library. Most of the buildings in the city and in neighbouring communities are low in height with only the Plaza, Postgirobygget and the highrises at Bjørvika considerably taller.

Oslo's architecture is very diverse. Architect Carl Frederik Stanley (1769–1805), who was educated in Copenhagen, spent some years in Norway around the turn of the 19th century. He did minor works for wealthy patrons in and around Oslo, but his major achievement was the renovation of the Oslo Katedralskole, completed in 1800. He added a classical portico to the front of an older structure, and a semi-circular auditorium that was sequestered by Parliament in 1814 as a temporary place to assemble, now preserved at Norsk Folkemuseum as a national monument.

Christiania, recently promoted to the status of a capital city, had practically no buildings suitable for the many new government institutions. An ambitious building program was initiated, but realised very slowly because of a strained economy. The first major undertaking was the Royal Palace, designed by Hans Linstow and built between 1824 and 1848. Linstow also planned Karl Johans gate, the avenue connecting the Palace and the city, with a monumental square halfway to be surrounded by buildings for the University, the Parliament (Storting) and other institutions. Only the University buildings were realised according to this plan. Christian Heinrich Grosch, one of the first fully educated architects in Norway, designed the original building for the Oslo Stock Exchange (1826–1828), the local branch of the Bank of Norway (1828), Christiania Theatre (1836–1837), and the first campus for the University of Oslo (1841–1856). For the University buildings, he sought the assistance of the renowned German architect Karl Friedrich Schinkel.

The German architectural influence persisted in Norway, and many wooden buildings followed the principles of Neoclassicism. In Oslo, the German architect Alexis de Chateauneuf designed Trefoldighetskirken, the first neo-gothic church, completed by von Hanno in 1858.

A number of landmark structures, particularly in Oslo, were built in the functionalist style, the first one being the Skansen restaurant (1925–1927) by Lars Backer, demolished in 1970. Backer also designed the restaurant at Ekeberg, opened in 1929. The art gallery Kunstnernes Hus by Gudolf Blakstad and Herman Munthe-Kaas (1930) still shows influence from the preceding classicist trend of the 1920s. Oslo Airport (by the Aviaplan consortium at Gardermoen was Norway's largest construction project ever.