Hjemmefrontmuseet, the Norwegian Resistance Museum, sits within the Akershus Fortress complex and tells the story of Norway's resistance to Nazi occupation during the Second World War. The exhibits are compact but strong on documents, photographs, underground newspapers, sabotage work, and the everyday risks taken by civilians.
It is a good companion to Akershus itself: the fortress gives you the setting, while the museum explains the occupation years in a direct, human way. Allow about an hour.