[Pornopung]

You have booked your flight already? You can try Aarhus, another good city in the north is Aalborg. It has a lot of students and a famous party street which is is quite busy every day and very cheap drinks. From Aarhus or Aalborg you could take a train to Hirtshals and then get the express ferry to Larvik, and then a train/bus to Oslo (although a flight from CPH can be had cheaper).