On the second day in Oslo we went to the amazing Opera House, and walked around on the roof. I haven't been there a lot, but I absolutely love the building, it's stunning. It was build by the architecture and landscape firm Snøhetta, which also designed the gorgeous library Alexandria in Egypt.
So if you ever find yourself in Oslo, I would recommend to take a walk on the Opera House, and you can see a beautiful view of Oslo on top of the roof. Beautiful view and architecture, you can't beat that.