This was one of the coolest places I went out while I was in Oslo! It features quite a schedule of live bands and even has a way for people to try out to have a free for all night. They also have a restaurant and a dance room for more hip-hop, trance folks. There is about a 100NK cover and the drinks can get expensive quick. But I saw the band Blodsbrøder who played lots of apparently quite popular Scandinavian rock.
Smuget is great but also give Will's a try just down from the Rådhuset plaza, or Centraal. Both were really good.
Dress Code: I saw that most people looked fairly well. Like they cared what they looked like before they stepped out the door. So bring some "goin out duds" and you will be fine. Nothing ratty.
First I have to point out that these places not are my favourtes. But they are always crowded with a lot of different people, specially Smuget. Rosenkrantzgaten is party-street with a lot of bars and discos
Galleriet - famous for people watching - always a lot of pretty and rich people there(or wannabees) commercial music
Smuget - a hang-out place for people in the music business(40-something crowd)
Horgans - an american restaurant and sportsbar for nice, rich good looking people (typical american bar, tv showing sport and commercial music) a small dancefloor downstairs- try the chicken-salad in the restaurant
Dress Code: At Horgans and Galleriet-think fashion. At Smuget-come as you are