ofcourse

IC from Stockholm to Gothenburg and from there the direct train to Oslo :)

Plan B quite long but you avoid the Highspeed train is Stockholm - Östersund - Trondheim - Oslo but this needs atleast 2Nights and a day :)

There is NO hi-speed at all on this route. But all long-dist trains anywhere in SE-also those seewulf mentions need to be reserved via sj.se. REad its FAQ on sj on how to do with passes. Fee is usually 65 SEK/ride, a little less for only IC.

Right now the Norwegian railway is doing construction work and there are no direct trains running between Stockholm and Oslo. Normally there would be both high speed trains and IC trains running.

There are regional trains between Stockholm - Göteborg, and Göteborg - Oslo.

But it’s not clear to me if a reservation is required on the Stockholm - Göteborg regional train. For example, the R 167 is leaving Stockholm Central at 09:14, and arriving in Göteborg at 14:00. 

Both the Rail Planner and DB navigator are telling me that a reservation is mandatory. However Interrail’s country info for Sweden says that reservations are optional.

@Nanja maybe the info about SJ regional trains is outdated.

I have looked at the SJ reservation site and the train from Stockholm at 9.14 does not require a reservation. The information in the planner is wrong. @Nanja 

It is only the SJ snabbtåg (high speed train) and the night trains that require reservations.