Oslo was the kicking off point for my whistle stop tour of Norway. It's a very small city, which you can get around easily on foot.
The main drag of Oslo is called Karl Johans gate, and it's the place to be seen on a Friday night. Everybody who's anybody gets out and has a walk down it.
I managed to catch a concert by military band in the Akershus Festning (a medieval fortress) on a Friday afternoon. Even the ducks on the duck pond seemed to like the music!