There are many, in Oslo...
Elm Street was mentioned in the german thread, they play a mix of rock, punk and metal. Depending on who's behind the bar
concerts from time to time with local bands...
Rock In, Traditional heavy metal, a lot eighties stuff... My favourite place. Not to crowded. Concerts from time to time...
Tut & Kjør (Honk & Drive
), They play sort of "best of heavy metal" Good atmosphere, but a lot of wannabees, crowded. Party til you puke...
Last Train, Oslo's oldest rock/metal pub, mostly crowded, concerts from time to time...
NYX, metal pub at 1. floor. Not to crowded, concerts from time to time with local bands. Popmusic at groundfloor, so don't be scared
So What, trendy place, rock and some heavy, crowded. Cool concerts from time to time
John Dee, large metal pub, often concerts (don't think that they are open when not concerts)
Rockefeller, same buliding as John Dee, larger, and only open when concerts...
Betong, only cool when concerts. Metalconcerts one ot two times a month...
Oslo Spektrum, large concert arena...
Most of theese places are located in the central city, near Karl Johan, main shopping street...
And several hundreds more pubs and restaurants. There might be some more playing good music, but I haven't seen more, yet
Lots of other attractions, depends on what you like to do.
I'll give you adresses and more information later if you want