Oslofjord, which runs in a north-south direction, is about 100 km long from the open sea to Oslo Harbour. About 30 km south of Oslo, where the fjord narrows at Drøbak, there is a threshold with a depth of 19.5 m. Inside (that is, north of this threshold) the maximum depths are 150 m.
Føyn, Ernst. 1958. "Hydrographic and biological investigations in Oslofjord by means of the densigraph and the oxymeter." Journal of Marine Research 17, (1). https://elischolar.library.yale.edu/journal_of_marine_research/912