Mona's place was perfect for my boyfriend and I. She was very accommodating and let us check in early as we arrived by overnight ferry. We walked everywhere in Oslo (trying to save money). But the public transportation from Mona's to downtown is extremely easy to use and very close.
The only thing I will say is that Mona's description is right. The apartment and shower are tight spaces so make sure you love the person you're staying with if you aren't traveling alone! I'd stay at Mona's again in a heartbeat.