Oslo - the oldest Scandinavian capital city was founded in early 11th century. It has some interesting sights:

  Akershus Fortress from the turn of the 13th and 14 century

  Castle Akerhus

  The Thor Heyerdahl Kon-tiki Museum

  The Munch Museum

  National Museum

  Vigeland Museum

  Vigeland Park with statues of famous sculptor Gustav Vigeland and many others

in the vicinity

  Norsk Folkemuseum is located at Bygdøy peninsula, approx 4 km west of downtown Oslo. Norsk Folkemuseum is one of the largest Open - Air Museums in Europe. You can admire more than 150 buildings from most areas in Norway. One of the many popular attractions is Gol Stave Church from year 1200.
This is a good opportunity to visit also these musuems: The Viking Ship Museum, The Kon-Tiki Museum, and The Fram Museum.