Their apartment is very centrally located and close to the main bus and train station (Oslo central). The apartment is clean and very cozy. I must mention that the apartment is on the third floor which is ground plus 3. Everywhere in Norway they consider ground floor as first floor.
There is a direct bus from just outside the bus terminal that takes you to their apartment. It's a quiet neighborhood but also very easily reachable.
Henrik and Fredrik are both amazing ask them anything you want to know about your stay at Oslo and they are more than happy to help.
I strongly recommend their apartment as we had an amazing time.