Oslo is a place name, the capital of Norway since about 1300 AD, but place names on kids are nothing new for American parents. Oslo is on-trend with other names that end in O, such as Otto and Arlo. In Norway this name is not used, but in the U.S. it has been used since about 2006, and was given to 16 boys in 2015.
Founded in 1049 by Harald Hardrada, the city Oslo had burned down in a fire in 1624, so the people moved it close to Akershus Fortress to rebuild. This was during the reign of Christian IV of Denmark, and then the city was renamed Christiania in honor of him. After 1925 the original Norwegian name was restored. It is currently a "global city" with a population estimated at 658,390.