We loved our stay at Nina's apartment.
It was clean and had all amenities we needed and more.
Location wise it is basically over the water behind the opera house which was about a 10-15 min walk from the train station. So ideal.
Also easy to get into town and sightsee.
There are a couple of bars, restaurants and a supermarket close to the apartment too.
Nina was very helpful and met us half way from the station. She also provided a travel cot and some lego for our baby which was lovely. We would definitely stay here again if in Oslo.