I had an excellent stay at Lisa's place ! She and her friend Nikolai are really reactive to speak with and to help you. The appartment was impeccable and contains so many things that you use in you everyday's life (more than mine actually) for cooking or in the bathroom. You can enjoy the couch during the day and the bed is up, like a cabin in the wood. The location is very central, close to shops, restaurants and the train station "national theatret" will bring you everywhere. You can get there in a 10 minutes walk, even in the snow.
I recommend Lisa's place. If I go back to Oslo while it is available, I will book it again.