Oscar Fredrik von Sydow (12 July 1873 – 19 August 1936) was a Swedish politician who served as Prime Minister of Sweden from 23 February to 13 October 1921.
Oscar von Sydow was the son of Henrik August von Sydow, a magistrate's secretary and Euphrosyne Maria Modin. He was born in Kalmar and raised in Norrland. In 1890, he passed his maturity examination and proceeded to study Law at Uppsala University. In 1894, he graduated with a degree in civil service.
In 1906, von Sydow was appointed Undersecretary of State at the Ministry for Civil Affairs, and in 1911 he became the Governor of Norrbotten County. In the governments of Hjalmar Hammarskjöld and Carl Swartz (1914–1917) he was Minister for Public Administration, during which he established the Unemployment Commission (arbetslöshetskommissionen). Between 1917 and 1934 he was Governor of Gothenburg and Bohus County.
Following the sudden resignation of Prime Minister Louis De Geer in 1921, the King had difficulties finding a candidate willing to form a new government as the elections were drawing near. After Hjalmar Branting declined upon being asked twice to become Prime Minister, the appointment went to Oscar von Sydow.
Von Sydow gave an ultimatum that he would not lead a powerless government and he demanded that the Social Democratic Party promise to support him in important finance and defence issues. Despite these agreements, all of the propositions made by the new government were voted down by the opposition. Von Sydow's lasting legacy as Prime Minister is for presenting the bill that abolished the death penalty in Sweden.
Von Sydow was Marshal of the Realm (riksmarskalk) from 1934 until his death in 1936.