We enjoyed staying at Hans's apartment in downtown Oslo. It was very convenient to the sights. The 30 bus line stops right outside and goes to the ship and folk museums. It's a few blocks to the opera house, central station, and Karl Johans gate. It was just a few more blocks to the Rådhus, aker brygge, and Grünerløkka neighborhood.
The apartment was great. Tons of light and all of the things we needed to cook, lounge, and sleep soundly. I'd recommend it to any couples coming to Oslo.