German cruiser Blücher

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. A German fleet led by the cruiser Blücher was dispatched up the Oslofjord to begin the German invasion of Norway, with the objective of seizing the Norwegian capital of Oslo and capturing King Haakon VII and his government. The fleet was engaged in the fjord by Oscarsborg Fortress, an aging coastal installation near Drøbak, that had been relegated to training coastal artillery servicemen, leading the Germans to disregard its defensive value. However, unbeknownst to German military intelligence, the fortress' most powerful weapon was a torpedo battery, which would be used to great effect against the German invaders.The fortress' armaments worked flawlessly despite their age, sinking the Blücher in the sound and forcing the German fleet to fall back. The loss of the German flagship, which carried most of the troops and Gestapo agents intended to occupy Oslo, delayed the German occupation long enough for King Haakon VII and his government to escape from the capital.

Håøya is an island in Oslofjorden in Frogn municipality, Norway. Its area is 5.6 km², which makes it the largest island in Inner Oslo Fjord. It is located between Oscarsborg and Gråøya. Søndre Håøya is a nature reserve area. White-tailed eagles nest on the southeastern part of the island. They returned in 2008 after an absence of 126 years. Visitors are permitted to harvest mushrooms and berries under 'freedom to roam' but other plants and flowers are protected. Goats graze the island during the summer. The island has a mooring jetty, camping areas, swimming beaches, public toilets, cabins for rent, and historic bunkers and buildings. The island was part of the defense of Oslo, along with Oscarsborg Fortress.

Oscarsborg Fortress (Norwegian: Oscarsborg festning) is a coastal fortress in the Oslofjord, close to the small town of Drøbak in Viken County, Norway. The best known part is situated on two small islets. The main artillery batteries are on the island Håøya and smaller batteries on 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 is best known for sinking the German heavy cruiser Blücher on 9 April 1940. In 2014, Oscarsborg Fortress was given protected status.

Fagerstrand is a village in the municipality of Nesodden, in the county of Akershus, in the country of Norway. It is south of Oslo on the peninsula of Nesodden. Fagerstrand has 1977 inhabitants (2006) making it the second largest village on Nesodden. "Fagerstrand" in English means "beautiful beach" or "fair beach". Fagerstrand is a small village with two grocery stores, two hair salons and one bar. There are two schools in Fagerstrand: Myklerud skole (elementary school) and Bakkeløkka ungdomsskole (junior high school). Fagerstrand is also the name of a reality show that was shot in Fagerstrand and aired on TV 2 during the first half of 2005. In the show, 12 couples built a house together. The couples voted each other out of the show until only two couples were left. Then the viewers would then decide who got the house valued at 3,000,000 NOK (ca. $500,000). Fagerstrand is also a very small area in the district of Høvik, which is a part of the municipal Bærum, across the fjord from Nesodden. This area sports a soccer field and a restaurant.

The Drøbak Sound (Norwegian: Drøbaksundet) is a sound at the Oslofjord narrows between Drøbak and Hurum. Outer Oslofjord, which is a term for the Oslofjord south of the Drøbak Sound until it meets the Skagerrak. Inner Oslofjord which is a term for the rest of the fjord, that starts in the Drøbak Sound northwards towards Oslo, where the fjord makes a turn and continues to the Bunne Fjord. The Drøbak Sound was previously guarded by Oscarsborg Fortress. During the German invasion of Norway on 9 April 1940, the German cruiser Blücher was sunk by the fortress. Oscarsborg Fortress has been converted into a museum and a hotel. There is a ferry from Drøbak. The subsea Oslofjord Tunnel at the Norwegian National Road 23 runs underneath the Sound and connects the east side and the west side of the fjord.

The Oslofjord Tunnel (Norwegian: Oslofjordtunnelen) is a subsea road tunnel which traverses the Oslofjord, connecting Hurum and Frogn in Norway. Carrying three lanes, the 7,306-meter (23,970 ft) long tunnel reaches a depth of 134 meters (440 ft) below mean sea level. The tunnel has a maximum gradient of seven percent. It acts as the main link connecting eastern and western Viken county, supplementing the Moss–Horten Ferry which runs further south. The tunnel is since 2018 a part of European route E134, until 2018 it was part of National Road 23. The crossing was originally served by the Drøbak–Storsand Ferry, which commenced in 1939. Plans for a fixed link were launched in 1963, originally based on two bridges which would connect to Håøya. Plans resurfaced in the early 1980s with the advent of subsea tunneling technology and the Oslo Airport location controversy, which proposed airports in Hurum, Ås and Hobøl. Even though Gardermoen was ultimately built as the airport, the tunnel had raised sufficient support to be built irrespectively. Parliament gave approval on 13 December 1996 and construction started on 14 April 1997. The tunnel was official opened on 29 June 2000 and was financed in part by a toll, collected by Bompengeselskapet Oslofjordtunnelen at a toll plaza in Frogn. In 2014, the manual toll station was replaced by an automatic station. Since 30 August 2016, it is free to pass through the tunnel. The tunnel was flooded in 2003 and 2008 and experienced a landslide in 2003. All of these incidents resulted in the tunnel being closed for weeks. There have been two major truck fires, one in 2006 and one in 2011. After the latter incident, the tunnel has been closed for heavy traffic exceeding 7.5 tonnes. In an effort to eliminate the problem, the Public Roads Administration has proposed building a second tube.

Drøbak is a town and the centre of the municipality of Frogn, in Viken county, Norway. The city is located along the Oslofjord, and has 13,409 inhabitants.

Heer is a village in Frogn, Akershus, Norway.

Frogn is a municipality in Akershus county, Norway. It is part of the Follo traditional region. The administrative centre of the municipality is the town of Drøbak. Frogn was established as a municipality on 1 January 1838 (see formannskapsdistrikt). The city of Drøbak was merged with Frogn on 1 January 1962. Frogn is located at the southern part of the peninsula between the main Oslofjord and Bunnefjorden. It borders Nesodden, Ås and Vestby.

Spro is a village in Nesodden municipality, Akershus, Norway. It is a former site of ice block extradition industry, and there are a number of man-made ponds created for this purpose. The village is located in southern Nesodden, between Fjellstrand in the north and Fagerstrand in the south. On the opposite of the Oslofjord is Nærsnes in Røyken.

Svestad is a village and a former oil and gas harbour in Akershus, Norway. Svestad is situated at the Oslofjorden side of the peninsula and municipality of Nesodden just south of Oslo.

Båtstø is a small village in Asker municipality in Akerhus county. The village is situated along the Oslofjord between Åros and Nærsnes in Røyken. The distance to Drammen is about 24 km and to Oslo about 38 km. The population of Båtstø was 228 as of January 2005.