There is no direct train on this route. There are trains to Göteborg (3 times per day) and from Göteborg to Malmö you take Öresundståg (no reservation needed - you have to search for trains with no reservations to get this trains showed) or SJ X (reservation needed). 

@AnnaB maybe you know about some works on this line that made no trains possible. 

If there are works, you have to run via other routes to Sweden like via Charlottenberg border or to use if the is a replacement bus for this (in Scandinavia, they often only cancel the train). Search on vy.no if the train Oslo-Göteborg is running. 

@Angelo I'll check for an update with my friend who works at SJ.

I searched now on vy.no and there are works on the line Oslo-Göteborg. You have to take the bus (not included in the pass) or the replacement bus Oslo-Rygge (included in the Interrail pass)

Here as example the ride from Oslo to Göteborg 30.07. at 13:31

@Angelo 

There will be busses on the Oslo-Göteborg line at least the whole of this year.