The simplest it can be to rent an apartmemt and be 15 min close to the city centre.
Great backyard and a nordic balcony view. The host is extremely friendly and has the sweetest lilttle dog. I had no internet and the host even gave me a portable wifi that I could carry around the city while I was sightseeing.
Affordable, simple and enough comfortable for everyone visiting Oslo from abroad. 4.5/5.0