Hans Petter's flat is perfect for one person or a couple, more could fit if they're friendly. It's very affordable, is in one of my favorite areas of Oslo, and has everything you need including an espresso machine. (But honestly, with both Tim Wendelboe or Supreme Roastworks within easy walking distance, why go through the trouble?) The key pickup process was a little different than what I've used in the past (Sharebox) but was entirely painless. The lighting is controlled by a Phillips controller so if you have the app on your phone you can control everything without having to get up. There's a bit of construction going on at the moment but didn't notice any noise when I was in the apartment. Hans Petter is also very responsive and assuaged my panic when the fire alarm went off at 02:30 one evening. I very much recommend his apartment!
As others have mentioned, there's a narrow spiral staircase that leads to the bedroom, while I didn't find it to be particularly treacherous, maybe skip that fifth pint at the Grünerløkka brygghus