Finishing up a 2 night stay here so I thought I'd share my experience. Hotels were $200+/night in the central area, so I ended up booking a very reasonable rate at the Radisson Oslo airport. It's a little confusing as there are 3 separate Radisson hotels on site at the airport, along with some off airport options, but I opted for one of the on site hotels after reading about the off site properties charging fees to take their airport shuttle bus.

There are convenient trains to/from Oslo Sentral station but the best value was taking the Vy regional train back and forth. Note that this is not the Flytoget airport express train which is significantly more expensive yet only a few minutes faster than the regional train. I was able to buy a 24 hr ticket for "all zones" (4N to 1) on the Ruter app for only 291nok, then travel on any Vy regional train between airport and Oslo. This also includes bus/tram/etc while you are in Oslo. The train station is 5 minute walk from the hotel..shorter if you stay at the Radisson Blu.

Of course,the downside is the lack of amenities in the airport location but with careful planning, it was worth the money saved. Radisson gives 10% off food for all members,so that helped a little as well when I ate at the hotel one night.