No, Oslo is in Norway and the official language is Norwegian. However, like most of Europe (Parisians pretend not to) they are fluent in English. It is best to learn some basic terms of Norwegian such as hello, please, thank you, and goodbye, but otherwise most people in Oslo will speak English.

People speak very good English, but Norwegian is the native language there.

I have many friends from Norway and they all learned English by 10 years old or younger and like Aaron said, you should learn simple things, just to be respectful. If you are going it is very very expensive!! Just a heads up

No, but a lot of people there (not all of them) will speak english if requested