The Action in the Oslofjord occurred late on 8 April 1940 in World War II. As Kampfgruppe 5 of the German invasion force proceeded towards Oslo, it encountered Norwegian defences in the Oslofjord. During a short exchange that resulted in the first Norwegian casualty of the war, the Germans managed to push through down to Drøbak Sound, where they would face more determined resistance.

The Battle of Drøbak Sound took place in Drøbak Sound, the northernmost part of the outer Oslofjord in southern Norway, on 9 April 1940. It marked the end of the "Phoney War" and the beginning of World War II in Western Europe.