Well two nights in Oslo will wipe out any savings!

My suggestion based on just doing this last week would be stay in the centre on your first night and then at the Radisson on the night before an early flight. You can see some room photos on my TR, rooms we stayed in were refreshed. This also details my hotel in the centre for my return which was more than adequate, cheap at 5 months out and only a 5 minute walk from the station.

Return fare on the express train is £30 so you don't want to be gong backwards and forwards all the time.