If you happen to be in Oslo on a Sunday I can highly recommend the Grand Cafe for Sunday brunch - in addition to great food, it's also the best value meal in Norway!! (It was a while ago that we visited the city, but I have fond memories)
For the airport we've stayed at Park Inn on a previous ex-EU (very good) and another stay at Radisson Blu coming up soon. Oslo itself is extremely expensive, and the airport prices quoted above are similar to the city from my experience.
Have a fun trip!