By night Oslo is a lively city, especially if you fuel up with the local liqour Aquavita which is made out of potatoes and herbs. Norwegians start their party day in Oslo with Champagne lunch, on a sunny day our favorite place is Bølgen&Moi or Champagneria. After lunch norwegians attend their vorspiel (pre-party) involving drinks at home, before they hit the streets again. If you talk to people at lunch you will probably get invited for a vorspiel.
If you are more of a quite type or prefer less drinking there are lots of other activities to attend to.