Holmenkollen is not only a skijumping place, if you have interest for skiing and history around skiing. This is the museum for you. The museum is located in the bottom of the famous skitower.
The museum contains 4000 years of skiing history. And is said to be the oldest of its kind in the world. The tower itself offers a great panoramic view.
Keep in mind that if you buy the Oslo pass, museums in Oslo are free.