I highly recommend Chris' place for your trip to Oslo. It is located conveniently on a metro line and a couple of buses serve it too, also during the whole night. The apartment is super cozy and can host up to 4 people, with 2 sleeping comfortably on a sofa bed. We loved the little touches Chris had thought of, like a fresh bunch of flowers and hand lotion. :)
Chris is a fantastic host, always available and very accommodating. Although we did not meet him in person, he did everything possible to make us feel at home. He sent us a very detailed and brilliant list of recommendations on what to do in the city, indicating also how to reach each thing from the apartment. We practically did not need to use any other guide book.
Look no further, this is the perfect place in Oslo. :)