Visiting Olso you must see Vigeland's Park. Gustav Vigeland is the most famous Norwegian sculptor. His works have fascinated me since I remember but when I saw them in real I was able to understand why. There are 212 sculptures in the park and in the highest point there's Monolit made of 121 figures. The whole park is amazing, making a strong impression and as only you have possibility of visiting Oslo it should be your main point to see:)
Have a nice day!