Mari's place is amazing! It's a 7 minute walk to the central station, and super close to all the amazing things Oslo has to offer. It was very clean and comfortable, with everything you need clearly well thought out by Mari (extra towels/pillows/sheets/supplies/etc.). It had a washing machine too, which was great, especially for such a compact space. It's definitely best for one or two people.
As others have mentioned, the tram is right outside the window - but it didn't bother me at all, and it isn't so loud that it woke me up once I fell asleep. Plus, if you want to be close to transit, you really can't get much closer than this.
I ended up meeting Mari for check-in, so just message her before your stay if you're concerned about picking up the keys as part of the check-in process. Mari is extremely kind and knows a lot about the city, so it's probably worth your while to meet her and tell her your plan anyways. And if you don't get to meet her, she has a with a list of activities to do around the city.
Overall, it was a wonderful experience for me, and a great value for a clean and enjoyable unit with a great location!