The Oslofjord (Norwegian: Oslofjorden) is an inlet in the south-east of Norway, stretching from an imaginary line between the Torbjørnskjær and Færder lighthouses and down to Langesund in the south to Oslo in the north. It is part of the Skagerrak strait, connecting the North Sea and the Kattegat sea area, which leads to the Baltic Sea. The Oslofjord is not a fjord in the geological sense - in Norwegian the term "fjord" can refer to a wide range of waterways. The bay is divided into the inner (indre) and outer (ytre) Oslofjord at the point of the 17 km long and narrow Drøbaksundet.
The entire population situated around the Oslofjord including Oslo is about 1.86 million, and the total population of all the counties situated around the fjord is approximately 2 million.
Each of the islands in the innermost part of the fjord has its own identity and distinguishing history. Among them are Hovedøya, Lindøya, Nakholmen, Bleikøya, Gressholmen, and Langøyene. These islands can easily be reached with the Oslo-boats from Vippetangen. Hovedøya is most famous for its monastery ruins, Gressholmen for its rabbits, Nakholmen, Bleikøya, Lindøya for their cosy cabins at the water’s edge, and finally Langøyene for its camping possibilities and pleasant beach.
In the summer there are boats of all sizes on the fjord, and it is possible to go kayaking, canoeing, fishing, and sailing. There are many activities for both adults and children.
The oldest settlements in the area surrounding the Oslofjord date from the Stone Age and the Bronze Age. It was here on the eastern and western shores that three of the best preserved Viking ships were unearthed. In historical times, this bay was known by the current name of the region, Viken (the bay).
Oslofjord has been an important body of water strategically due to its proximity to Oslo. During WWII, there were German installations at several points on its coastline. One installation in Hovedøya held 1,100 Wehrmacht soldiers and later women deemed Nazi collaborators at the National Internment Camp for Women in Hovedøya.
Second World War 
The fjord was the scene of a key event in the German invasion of Norway in 1940. The invasion included a planned landing of 1,000 troops transported by ship to Oslo. Colonel Eriksen, Commander of the Oscarsborg fortress near Drøbak, mainly maintained for historical purposes, sank the German heavy cruiser Blücher in the Drøbak narrows.
The fortress's resistance blocked the route to Oslo, thus delaying the rest of the group long enough for the royal family, government, parliament, and national treasury to be evacuated. The result was that Norway never surrendered to the Germans, leaving the Quisling government illegitimate and permitting Norway to participate as an ally in the war, rather than as a conquered nation.
The Oslofjord is one of the nine venues of the Class 1 World Powerboat Championship.
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