from Wikipedia Oslo:
Oslo (called Christiania from 1624 to 1878, and Kristiania from 1878 to 1924) is the capital and largest city of Norway. It is also a municipality and a county of its own. It is the third-largest Scandinavian city, after Copenhagen and Stockholm, and it forms the third-largest urban area in the region, after Copenhagen and Stockholm. It is arguably the world's most expensive city in which to live.
The city of Oslo was established as a municipality on 3 January 1838 (see formannskapsdistrikt). It was separated from the county of Akershus as a county of its own in 1842. The rural municipality of Aker was merged with Oslo on 1 January 1948 (and then transferred from Akershus county to Oslo county). Furthermore, Oslo shares several important functions with Akershus county.