As of 1 January 2016, the municipality of Oslo had a population of 658,390. The urban area extends beyond the boundaries of the municipality into the surrounding county of Akershus (municipalities of Asker, Bærum, Røyken, Rælingen, Lørenskog, Nittedal, Skedsmo, Ski, Sørum, Gjerdrum, Oppegård); the total population of this agglomeration is 942,084.
The city centre is situated at the end of the Oslofjord, from which point the city sprawls out in three distinct „corridors“—inland north-eastwards, and southwards along both sides of the fjord—which gives the urbanized area a shape reminiscent of an upside-down reclining „Y“ (on maps, satellite pictures, or from high above the city).
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, 130 km2 (50 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).[citation needed]
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.
Source: Wikipedia