Perminalen Hotel is located in the city centre of Oslo and offers nice and cheap rooms/dorms (e.g. rooms with 4 beds, a shower and a toilet).
24/7 someone is sitting at the reception desk and if you don’t want to let all your valuables on your (shared) room you could put them there when you’re leaving and get them back again when you’re coming back.
The rooms are beeing cleaned each morning.

They also offer breakfast which is actually not too bad.

In my opinion it’s hard to find a better place to live for some days in Oslo. Many places, sights and shops are within walking distance, including the subway (called T-Bane) and the central station/bus station.

They also have a WLAN free to use for guests - I couldn’t use it on my room, because the signal was to weak, but it’s not the biggest problem to find a place to sit inside the hotel where the signal’s strong enough to check your e-mails.