One of the most interesting museums in Oslo is free!  The Oslo City History museum in Vigeland Park even has a movie in English about a thousand years of Oslo history.  There are many rooms of interesting displays.  The portraits of King Christian IV of Denmark and Norway are  interesting to see, and are very different from portraits shown in Denmark.  Danish portraits don't show his nose to be the size of the rest of his head.  The museum is a popular destination for groups of well-behaved schoolchildren.