The Oslo Opera House is probably Norway's most famous piece of architecture. It's the second most famous opera in the world (in my opinion and also in real life). Designed by world-famous design group Snøhetta, this modern marvel sits at the head of the Oslofjord, with its sleek marble and granite exterior. You can walk up on the roof, and it is the #1 touristy thing to do in Oslo (and it's totally worth it!)