I haven't been there yet, not on the inside, but I'm planning on going pretty soon, for an opera or a ballet performance. Oslo isn't that far,and it's absolutey possible to go there for a show and be back the same night. We've only been back in Norway for about 7 months, and there has been a lot of things to organize, but hopefully soon there will be a possibility!
These pictures were taken in February, on a icy cold day, and my fingers were half dead from the effort. The Opera is sitting on the water, the Oslo Fjord, which was frozen at the time.
As you can see from the pictures, it's possible to walk on the roof, all the way to the top. Right now, signs were up asking people not to, because of the slippery ice and snow, but nobody seem to care too much about the risk of falling.