We had a great time at the little Annex!
Thanks to quick responses by Rannveig&Johan and some superb tips we had no problems at all arriving/ getting around.
The renovated Annex is super cute, very well equipped for self-catering (better than your average holiday home, for sure) and rather spacious - just perfect for two people.
The bed was very comfy and all was sparkling clean and tidy.
The area has a few shops, supermarkets, and a bar or two nearby, but it's only a few minutes to the city center by bus (or about 30 minutes walking through a park/by the river, which we highly recommend)
If we come back to Oslo, this is certainly where we will stay again.