Ytre Oslofjord (Norwegian literally "Outer Oslo Fjord") is that part of Oslofjord which is south of the threshold of Drøbaksund (literally Drøbak Sound), the strait  where the fjord narrows between Drøbak and Hurum, Norway.
Ytre Oslofjord refers to the southern portion of Oslofjord bounded by the Drøbaksund Strait, a narrow passage connecting Drøbak and Hurum in Norway.