Hey Maureen,

because of Corona there are no cross-border trains from Norway to Sweden and vice versa right now and it is not known until when they will return… (likely not this summer anymore! unfortunately)
There are some buses crossing the border (check on sj.se or vy.no). Also you could check the ferry routes (depending on your budget)

Safe trip

Martin

Hello,
there is a Bus connection (not included) from Oslo to Gothenburg and from there you can take the train to Copenhagen. The connection is shown on sj.se .

You can take discounted ferries from Oslo to Hirtshals or Kiel (Germany) or book a ticket on the not discounted DFDS Ferry from Oslo to Copenhagen A British traveler paid once 2$ for the ferry to Copenhagen I paid arround 20€ here a video

Does anyone know how to book a discounted ferry?

Need to get from Oslo to Belgium.

Ferry to Kiel would be a good start. There are no trains through Göteborg, only buses this summer.

But, where to book the ferry.. there are these ferry booking websites, are these expensive, where can I apply Interail discount? 

Wondering what the cost is for 1 adult +1 child versus a looooong day of trains Oslo to Copenhagen and next day on to Hamburg,...

@maxlenssens 
Ferry from Oslo to Kiel offer no discount :/

As you wanna go to Belgium you may catch the new Ferry between Norway and Emshaven (Netherlands) there is no discount but brings you quite direct where you wanna go :)
Kristiansand - Emshaven ferry