Reservations are mandatory on both trains.

For Bergen - Oslo, you can book at a station in Norway or call Entur (add country code +41 for Norway). Reservation in Norway is free of charge if you have a 1st class pass and book via Entur.

For Oslo - Stockholm, you can book on the SJ website.

Entur are excellent and they email the reservation s to you. It’s a good idea to print them if you can but the train staff are fine with details on your phone. Ring early or late Norwegian time to avoid a long phone call. If you have a first class pass tell the operator so you’ll be booked in the premium carriage with the free tea and coffee! As already said reservations are free. Norway is fabulously expensive so these are good perks. I’m writing to you from Norway where I’ve been for the last three weeks. The long distance trains are absolutely superb but you must reserve 

ENtur is a neutral organisation (formerly called Rutebok) that handles all ticketing (also the machines in the stations) and info on all trains on NO since when they started to contract out trains.