Hans Oster (1888-1945) was a German Army officer in the First World War, a staff officer during the interwar period, and rose to become the main operations officer of the Abwehr military intelligence agency and a major figure in the German Resistance. He was dismissed for anti-Nazi activities, and eventually executed at Flossenburg Concentration Camp.
Oster committed to subvert the Nazis on 7 November 1939, when he gave the German plans for the invasion of the West to a Dutch military attache. "There is no going back after what I have done. It is much easier to take a pistol and kill somebody; it is easier to run into a burst of machine gun fire than it is to do what I have done." While the information he gave was correct, Dutch intelligence did not take the warning seriously; Hitler changed the invasion date twenty-nine times. He is believed to have been a major intelligence source for the Soviet Lucy Ring, as well as participating in efforts to rescue Jews. 
In 1941, when Artur Nebe was assigned to command an Einsatzgruppe, he initially declined the assignment, asking to go to the International Police Commission. Ludwig Beck and Oster persuaded him to go to the Einsatzgruppen to gather information.
On 5 April 1943, Manfred Roeder, an Army investigator into the Abwehr and the Red Orchestra, first discovered Oster's activities as part of a search at Abwehr headquarters, originally focused on the activities of Hans von Dohnanyi at the Vatican. Roeder arrived, with a warrant, along with Gestapo investigator Hans Sonderegger.
As Dohanyi's safe was being searched in the presence of Oster and his chief, Admiral Wilhelm Canaris, with poor tradecraft they fumbled papers that Roeder and his Gestapo associate demanded. The papers were not utterly incriminating, showing Oster had been in correspondence with Dietrich Bonhoeffer, but gave the investigators a basis to look more deeply.  They discovered Oster's activities and forced his dismissal, initially as house arrest. 
Sonderegger, in September, forced open a safe, at the military headquarters at Zossen, containing much more incriminating information. Dohanyi had long demanded the files be destroyed, saying "Every jotting about the business is a death sentence", but Canaris declined, for uncertain reasons. Beck may have wanted them preserved for history, Canaris may not have felt safe in destroying them, or it may have been an oversight.
Oster was under arrest before the 1944 assassination attempt against Hitler, but was implicated and certainly was aware of the general conspiracy. Along with Canaris and Dietrich Bonhoeffer, he was shot at Flossenburg Concentration Camp in 1945.
- Anne Nelson (2009), Red Orchestra: the story of the Berlin underground and the circle of friends who resisted Hitler, Random House
- Mark A. Tittenhofer (declassified September 1993), "The Rote Drei: Getting Behind the 'Lucy' Myth", Studies in Intelligence, Central Intelligence Agency 13 (3)
- Michael Leonard Graham Balfour (1988), Withstanding Hitler in Germany, 1933-45, Psychology Press, pp. 163-164
- Gerald Reitlinger (1989), The SS, alibi of a nation, 1922-1945, Da Capo Press, p. 292
- Anthony Cave Brown, Bodyguard of Lies: The Extraordinary True Story Behind D-Day, pp. 302-304
- Michael Mueller, Geoffrey Brooks (2007), Canaris: the life and death of Hitler's spymaster, Naval Institute Press, pp. 252-253