Staying at Sigvart's place was great. We were given very clear instructions on how to get to the house, and how to get in. We caught the bus to the apartment, which took around 10 minutes from Central Station. I made use of the fully equipped kitchen and even with 4 people sharing the apartment, there was enough room for everyone. We used the fold out couch, which was surprisingly comfortable, although if you needed to use it for a longer stay, the novelty would wear off after a couple of nights.
There is a supermarket literally 3 floors beneath the apartment, as well as a barber, an Italian grocery store, a cafe and a ramen shop (which has great reviews)
Oslo is such a wonderful town with so much to see. The last thing you want to do is spend an unnecessary amount of time getting to and from accomodation. Because of the central location and easy accessibility, as well as the professionalism and friendliness of the host, I'd recommend Sigvart's place to anyone visiting Oslo.