The name Oslo is a boy's name of Norwegian origin. Oslo and is often added to lists like Unique Baby Names: Unusual and Rare and discussed in our forums with posts like "Boys Drop 2 Add 1 pick a favor that can't be dropped".
From the experts:
Oslo, the capital city of Norway, makes for a somewhat clunky boy's name. Might be better to think about the city's former name, CHRISTIANIA or KRISTIANIA for your daughter.