Kim, if you're planning to stay a few days I'd stay somewhere centrally located so you can walk most places. The airport is about 25 miles away.
There is an express train that runs right to the airport every 10-20 minutes - (it takes about 20 minutes - you just insert your credit card in a terminal before you enter) so its easy to get back to, but I think you'd enjoy touring Oslo more if you based yourself in town.
