The Viking Ship Museum and Norse Folk Museum are both located in the Bygdøy area of Oslo, easily reached by ferry (in the summer) or by bus (number 30 all year round) and within walking distance of each other.
The Viking Ship Museum displays three viking ships found in burial mounds throughout Norway along with information about the people and objects buried along with the ships. The museum itself is small but has plans to expand in the next few years.
The Norse Folk Museum is made up of an indoor museum devoted to the development of folk art and culture in Norway as well as an open air museum with buildings showing life in Norway from the Middle Ages to the twentieth century. It's much bigger than I expected so make sure to allow yourself plenty of time to wander through the buildings.