The capital and chief port of Norway, on the south coast at the head of Oslofjord; population 839,423 (2007). Founded in the 11th century, it was known as Christiania (or Kristiania) from 1624 until 1924 in honour of Christian IV of Norway and Denmark (1577–1648).
Definition of Oslo in US English:
Oslo
proper noun
The capital and chief port of Norway, on the southern coast at the head of Oslofjord; population 839,423 (2007). Founded in the 11th century, it was known as Christiania (or Kristiania) from 1624 until 1924 in honor of Christian IV of Norway and Denmark (1577–1648).