Hans Oster served in the First World War as a General Staff officer and was later among those allowed to remain in service in the Reichswehr, which he left in 1932 for personal reasons. Hans Oster was married to Gertrud Knoop, with whom he had a daughter and two sons.

From October 1933, Oster served as a civilian employee, and from 1935 at the rank of lieutenant colonel, in the Reich War Ministry's counterintelligence department (which later became the Office for Foreign Affairs/Counterintelligence in the Armed Forces High Command). He became head of the central department of the counterintelligence office and was promoted to colonel in 1939. Oster wanted to overthrow the regime by assassinating Hitler as early as 1938. He assumed key duties in every plan for a coup attempt and was the focal point of the resistance group in the counterintelligence community. In 1940, he informed the Dutch military attaché in Berlin of the impending invasion of the Netherlands by German troops. In 1941 he was promoted to major general and appointed chief of staff of the Office for Foreign Affairs/Counterintelligence of the Armed Forces High Command. In the fall of 1942, he supported his colleague Hans von Dohnanyi's plan to enable thirteen Berliners under threat due to their Jewish origins to escape to Switzerland. The plan was known within the counterintelligence service as "Operation Seven." Oster was forced to resign in April 1943 because of alleged foreign-currency dealings, after which he remained under Gestapo surveillance. He was designated to become presiding judge of the Reich Military Court following a successful assassination of Hitler.

Hans Oster was arrested the day after the unsuccessful coup of July 20, 1944. On Hitler's orders, he was murdered in Flossenbürg concentration camp on April 9, 1945.