You’ll find all the information you need on the reservations section of this site!

For Copenhagen-Oslo, you normally don't need reservations for the direct route, i.e. with one change in Göteborg. Various planners are giving different results, some with a very long detour, e.g. SJ sends you via Stockholm (!) and DB proposes 4 changes and 3 trains with mandatory reservations and it looks like they don't know the direct trains Göteborg - Oslo. Entur say Göteborg-Oslo is running but Vy (the operator) doesn't. I'd first ask Vy what's happening on 11 March.

Oslo-Stockholm can be booked on sj.se but it looks like 14 March is not open to book yet. The same is true for Stockholm-Copenhagen on 17 March.

Seats can be reserved from Swedish Rail online https://www.sj.se/en add interrail as discount.

Booking window is 2 months so you are a little early.

Oslo - Stockholm and Stockholm should be available within the next week, reservation cost at 62SEK (€6) per seat.

Copenhagen - Oslo is more of an issue. The direct route is normally via Goteborg (and does not require reservations) but nothing on the route north of Goteborg is showing (except scheduled buses) so possibly engineering works.

The scheduled buses show up on the se website but they do not accept interrail passes, the price shown will be the full price.

Reservations with SJ are at the moment possible until the 12th of March, night trains until the 11th.

@Al_G 

No one, not even SJ, knows when reservations in Sweden after 12th of March will open. It will definitely not be next week. There is no booking window for tickets at SJ. They normally release tickets 4 times a year.

OMG… it seems they're not handling that any better than Trenitalia :(

SJ are not to blame. The problem is that SJ, and other train operators, don't get information about planned construction works from the Swedish Transport Administration (Trafikverket), which is the Government agency responsible for the long-term planning of the transport system, until very late.