I strongly recommend staying at Anders' place. The room was very clean and the bed was comfortable. Anders and Joanna are helpful and provide guests with precised instructions on how to get to the place, open the doors, etc. Due to central location I had expected noise from the outside but it was surprisingly quiet. Although we could hear some noises from the inside of the building, it was all quiet during the nightime.
The place is located about 20 min walk from the central station, which gives you a chance to see some of the well known sights in Oslo, so it's perfect for a short stay.