The city that we know today as Oslo was founded in AD 1040, as Ánslo, and became known as Oslo. In 1624 it burnt down; a new city was built nearby, and named Christiania in honour of King Christian IV (of Denmark and Norway).
An official spelling reform in 1877 led to the city being known as Kristiania by the national government and on official documents, but the city authorities only adopted this name in 1897.
Meanwhile, the name Oslo had survived as the name of a village that grew up on the site of the old ruined city, which had become a suburb of the newer city. In 1925 that city was renamed Oslo.
© Haydn Thompson 2020