Grünerløkka is probably the best place to be for young adults visiting Oslo. Staying at Henrik’s place was easy and delightful. We had a problem with the heaters, but we got a standalone one the next day. The disherwasher was unfortunately broken, but the water in Norway makes washing up easy. This is a central place, food and coffee for all hours (except the late night time week days) around the corner good quality (or cheap if you’d like that), the tram takes you into the center in 10-15 min and then you can choose to go wherever.
I’d recommend Holmenkollen ski tower and museum for a great view over Oslo and the Oslo fjord, plus the forest.
And I am recommending to stay at Henrik’s place.