On this page there is a link to information about accessibility in the Eurail countries,
https://www.interrail.eu/en/support/interested-in-interrailing/travelling-with-a-caregiver
If you need to sit in your chair during the travel, at least in Sweden, you need to make a reservation for a wheelchair place. Please note that due to construction works all trains from Oslo go via Gothenburg. There are a few direct trains per day from Oslo to Stockholm but most connections are by between Oslo and Gothenburg.
You should be able to make all reservations at sj.se
https://www.sj.se/en/kop-resa/sok/resa/fran-station/till-station/datum
When you add the traveler you must enter “Interrail or Eurail” as “Card Category” under “Add SJPrio/Period ticket. As “Card number” you enter E and 8 digits, not matter which.
When you have found the train you want to travel with you should under “seat requirement” select “Wheelchair space”.