My stay with Marthe was amazing. Her apartment is very close to the Royal Palace (about a 5 mins walk), and the metro/train station Nasjonalteateret (if you are taking a train from the airport buy a ticket to this station as it is a lot closer than from the Central Station and you dont have to walk or pay extra money for transport to get to the apartment). It is totally walkable from the Nasjonalteateret station to Marthe's apartment.
The apartment was very clean and everything was as described and pictured here on airbnb. The apartment is really beautiful, I really enjoyed the simple and minimalistic design etc.
Marthe was a very helpful host and gave me some valuable information about Norway and was ready to help me find alternative transportation to Bergen (there was a real chance my train might get cancelled because of a political strike). Everything went very smoothly, from getting the keys to my departure.
Thank you to Marthe for making my stay in Oslo such a great experience!