Oscarsborg Fortress is a coastal fortress situated on two small islets in the Oslofjord, near the village of Drøbak in Norway. The fortress was built in the late 19th century to protect Oslo and its surrounding areas from possible attack by sea. The fortress played a key role in the defence of Norway during the German invasion in 1940.

The main fortifications on the islands are the Kongsvinger Fortress, the Middle and the South Fortresses. The Kongsvinger Fortress was built in the 1850s and was the first fortification on the islands. The Middle and South Fortresses were built later, in the early 20th century. The main armament of the fortress is the two 28 cm guns on the Middle Fortress.

During World War II, the fortress played a significant role in the defence of Norway. On the night of April 9, 1940, a German naval force entered the Oslofjord with the intent to capture Oslo. However, the fortress fired its guns, sinking the German cruiser Blücher, and damaging other ships, which delayed the German invasion and allowed King Haakon VII and his government to escape to Britain.

Today, the fortress is a popular tourist destination and is open to the public. Visitors can explore the fortifications, visit the museum, and enjoy scenic views of the Oslofjord. The fortress also hosts various events and concerts throughout the year.