In reply to gilliesp:
Looks like the only direct option these days is Norwegian to Gardermoen on Tuesdays, Thursday and Sundays. Have a look at skyscanner.net to see the various other options.
We used Norwegian when we went to Rjukan a few years ago. ISTR that we found the costs pretty closely matched cf Ryanair once we'd factored in the cost of putting winter gear for a week for each of us in the hold. The timing of the Norwegian outbound flights looks better now than it was when we went, though the return flight is at sparrow's fart so you'd probably need a doss near the airport for the night. We stayed at the Gardermoen Airport Motel & Apartment I think - which was more like a decent bunkhouse than what we'd call a motel, really. We returned the rental car to the airport rental office the night before and got the shuttle bus to the terminal the next morning from the posh Clarion hotel next door.