I stayed at Marina's for a week in September. Marina was only there in the start and in the end, but she was super nice and very helpful. The apartment is pretty close to the center. You could walk to the main station, but it's still nicer to take a bus, and there's lots of them!
My room was very big and calm so it was ok spend a lot of my free time there. But there is also a kitchen that you sometimes share with other guests and I got to meet a lot of interesting people and have some nice chats!
All in all it was a good experience and I would recommend it to anyone who wants to stay in Oslo, especially for a longer period.