Oscarsborg Fortress (Oscarsborg festning) is a coastal fortress in the Oslofjord, close to the small city of Drøbak. The fortress is situated on two small islets, and the mainland to the west and east, in the fjord and was military territory until 2003 when it was made a publicly available resort island. The fortress became famous during World War II for the 9 April 1940 sinking of the German heavy cruiser Blücher.
The narrows at Drøbak, called Drøbaksundet is a natural point for the naval defence of Oslo, the capital of Norway. The first defences were constructed during the reign of Christian IV of Denmark and Norway and were ready in 1644. The fortifications were however not involved in battle during the Hannibal War.
After the war the fortifications were dismantled, and only resurrected for a short period during the 1814 war with Sweden.
Around 1830 the discussion started for a renewed fortification of the Drøbak Narrows and the first stage was ready in 1848, the next in 1853. The name of the fortress was given by royal resolution on 23 August 1855 after a visit by the Swedish Norwegian King Oscar I.
By the end of the 19th century the art of war developed rapidly and the new fortress was soon obsolete. The tension was also growing between the two countries in the union and so the Norwegians decided to upgrade the fortress. From 1890 new improved German guns were installed, an underwater barrier was built in 1874-79, and an underwater torpedo battery was constructed. The main armament was three 28 cm calibre guns manufactured by Krupp. There was also a number of guns with smaller calibres (15 cm and 57 mm) on the mainland. An underwater barrier went from the main islet of Kaholmen and southwest to Hurum on the western side of the fjord, thus making it impossible for large vessels to sail west of the fortress.
At Oscarsborg the torpedo battery is a concrete construction inside a cave mined into the rock of the North Kaholmen island. Two torpedoes are loaded side by side, in an open steel frame. Then the frames are lowered like an elevator down in the water to the tunnels below. When fired, the torpedo's own compressed air engine was started and it propelled itself. The battery has three torpedo tunnels which could fire six torpedoes without reloading and a total of nine torpedoes were stored and ready for use.Each weapon carried a 100 kg TNT warhead and targets were spotted from an observation bunker just above the battery.
During the occupation of Norway German forces were stationed at Oscarsborg.
After the deactivation of the last weapons systems, the remaining military activity on Oscarsborg consisted of the Coastal Artillery officer training programme. The officer school was officially shut down on 28 June 2002.
The fortress is now largely a civilian resort and attraction, open for visitors. The scenic surroundings is much used for conferences and excursions. Visitors take a short motor launch trip from Drøbak.