Intriguing! I'm not too knowledgeable on this timeperiod but from my impression right now everything is still OTL and you are just setting things up. Is that correct or have I fully missed the POD?
How did Oslo grow so exponentially? Norway’s population after the plague was roughly 150 000 people, so this über-Oslo would be quite massive, it’d also be by far the largest city in all of the Kalmar Union. In otl Oslo’s population peaked at 3 500 before the plague and dropped to under 1 000 after it.