The Oslofjord is an inlet in southeastern Norway. The 120-kilometre (75 mi) fjord begins at the small village of Bonn in Frogn Municipality and stretching northwards to the city of Oslo, and then curving to the east and then south again. It then flows south to an imaginary line running between the Torbjørnskjær Lighthouse and Færder Lighthouse where it becomes part of the Skagerrak strait. The Skagerrak connects the North Sea and the Kattegat sea area, which leads to the Baltic Sea. The Oslofjord is not a fjord in the geological sense, but in the Norwegian language, the term fjord can refer to a wide range of waterways including inlets such as this one.

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. The city of Drøbak was merged with Frogn on 1 January 1962.