Born 1888 in Dresden, Germany, the son of a protestant parishioner.
Served as a General Staff officer in the German Army during World War I. Was a member of Reichswehr, the limited German Army allowed as part of the Versailles Treaty. Served in the German War Ministry beginning in 1933, later becoming a Colonel, where he headed the Second Department of the Abwehr (oversaw records of German intelligence agents).
A man of moral character, Oster felt disdain for the Nazi movement and contempt for Nazi Fuehrer Adolph Hitler. Aligned himself with other high-ranking officers who felt the same (including spymasters Wilhelm Canaris and Erwin von Lahousen). Clandestinely aided in the escape of Jews from Germany by funneling money to them on the pretense of sending them out to spy in Switzerland.
Manfred Roeder of the Reich Military court launched an investigation into Army personnel working against the Nazi Party and discovered notes indicating Oster's involvement in helping the Jews escape as well as others which detailed attempts to negotiate a separate peace negotiation with with the German Army and the Allies (brokered through the Vatican). Canaris was forced to dismiss Oster.
Oster continued his activities, attempting to oust Hitler. He participated in the ill-fated plot to explode a bomb on a plane in which Hitler would be a passenger as well as the attempt to detonate a bomb during a meeting (both attempts failed). As conspirators were rounded up, Oster was at the top of the list and was immediately arrested.
Oster was executed on April 9, 1945 at the Flossenburg concentration camp.