Perfect location just a couple of blocks from the train station. This is the perfect hub from which to explore the city center and beyond. We walked everywhere and nothing felt too far. The Royal Palace is about 10 mins on foot. If you continue up Bogstadveien into Majorstuen you will find high end stores and Oslo Mikrobryggeri. The apartment is only a 15 minute walk from the south end of Grünerløkka, with plenty of stops like the Oslo Street Food hall, Tilt video game bar, and the Crow Mikrobryggeri along the way which make the walk absolutely worth it. We also highly recommend seeing the outdoor Folkemuseet and the incredible Vikingskiphuset as well, although those are about a 300 NOK (30 USD) taxi from the apartment.
Anyway, if you're thinking about visiting Oslo then DO IT and if you want a great home base while there, stay here! We didn't spend much time in the apartment but it was comfortable, perfectly located, private, quiet, had a nice patio, and the hosts are great communicators who are always available by text or phone.