This is Frognerseteren, an old building from the 1890's, built in the Dragon style of the époque. There is a restaurant/café there, and the building is used for weddings and parties. Lots of tourists visit to see the fantastic view over Oslo. Frognerseteren is on top of Holmenkollen, 435 meters above the city.

The first photo: Lothiane
Second photo: Mark Wilkinson