This is the 6112th most frequent Japanese word.
オスロ
Oslo (capital city of Norway).
In this sentence, 'オスロ' is used as the proper noun to refer to the city of Oslo, serving as the subject of the sentence.
オスロはノルウェーの首都です。
Oslo is the capital city of Norway.
Here, 'オスロ' is used again as the proper noun to denote the location where someone resides. It is not the subject but an object within the prepositional phrase 'オスロに' indicating location.
私の友人はオスロに住んでいます。
My friend lives in Oslo.
'オスロ' in this sentence is part of the locative expression 'オスロで', specifying the location where the activity of sightseeing will occur.