My wife and I spent 3 nights at the apartment and it was a very convenient place to be located, with public transport, restaurants and the train station all within easy walking distance.
The apartment itself is tidy, cosy and had everything we needed. It was unexpectedly warm while we were there (32 deg celsius max temp) and the apartment did get warm at night. However the windows do open a bit and that did help. The bed is also in a room without windows which makes it stuffy in the heat but on the positive side, it helps keep the light out in summer when the sun sets around 10:30 and rises again at 2.
We were an hour late for check-in due to a delayed train from Bergen but Firas arranged for his sister to meet us at 11:30pm with the keys and she let us in without much fuss.
I would happily recommend this place as a fuss free base for exploring Oslo.