Between Oslo and Stockholm there are only a few trains per day. Slow trains, I think only 1 or 2 on the weekends, but not for the entire route but only some leg. The problem is to come over the border. Maybe you can take a train to Göteborg and change there for a train to Stockholm.  

@Inka 

What date do you plan to travel?

There are lots of construction works on the line between Stockholm and Oslo this summer so there will not be any trains going the whole way. There will be replacement busses for part of the way. I'll check and get back.

The best way to look for information on trains in Sweden is SJ.se. The Interrail/Eurail planner is only updated once a month and thus not reliable. 

Due to the construction on the direct route from Stockholm to Oslo, the connection is re-routed via Gothenburg. There are some trains going the whole way, but otherwise there is a buss from Gothenburg to Oslo. I need to check if the Interrail/Eurail pass is valid on the buss.

Thanks! We're taking a bus now :)

Have you checked that the Interrail pass is valid on the bus? Or are you buying a normal ticket?

@Inka 

Interrail is only valid on the direct trains between Stockholm and Oslo, see picture below. For connections where you change to bus you pay full price.

I looked at the trains on the 14th of July.

@Inka

Interrail is only valid on the direct trains between Stockholm and Oslo, see picture below. For connections where you change to bus you pay full price.

I looked at the trains on the 14th of July.

 

I know :)

We paid extra because we def need to travel on a specific date.