Oslo is a lovely city but food and drink are very expensive in Norway. There are lots of thigns to do, though. You can go and see Vigealnd's weird sculptures in the Frognerpark, the Akershaus fortress, and there are several museums a short ferry ride away on the Bygdoy peninsula, where you can learn about Amundsen, Viking ships and more. I also found the Holmenkollen ski jump a fascinating visit, though that probably isn't to everyone's taste. Finally, there are plenty of opportunities to go for a trip down the Oslo fjord.
I spent a week there and I didn't see everything. I can't compare it to the other two cities though, as I haven't been to either Vienna or Bratislava (yet).