Hovedøya is an island located in the Oslo fjord. The name derives from Norse word hǫfuð, meaning height. It probably got this name from a large hill on the Eastern part of the island where there used to be a military watch tower in Medieval times.
Hovedøya has a rich and varied history; from being a strategic military base in Viking times, to a monastery in the Middle Ages, to a quarantine station in the 1800's. Used as an internment camp for women who had consorted with Nazis after WWII, this camp was later used for temporary housing as there was a great shortage of houses after the war.
Nowadays the island is used mostly for recreational activities; beaches and art galleries, and some times puppet shows.
You can take a ferry from Vippetangen to get there.