He was a key planner of the Oster Conspiracy of September 1938. Oster was arrested in 1943 on suspicion of helping Abwehr officers caught helping Jews escape Germany. After the failed 1944 July Plot on Hitler's life, the Gestapo seized the diaries of Admiral Wilhelm Canaris, the head of the Abwehr, in which Oster's long term anti-Nazi activities were revealed. In April 1945, he was hanged with Canaris and Dietrich Bonhoeffer at Flossenbürg concentration camp.
Oster was born in Dresden, Saxony in 1887, the son of an Alsatian pastor of the French Protestant Church. He entered the artillery in 1907. In World War I, he served on the Western Front until 1916, when he was appointed as captain to the German General Staff. After the war, he was thought of well enough to be kept in the reduced Reichswehr, whose officer corps was limited to 4,000 by the Treaty of Versailles. However, he had to resign from the army in 1932, when he got into trouble because of an indiscretion during the carnival in the demilitarised zone of the Rhineland, where Reichswehr officers were prohibited.
He soon found a job in a new organisation which Hermann Göring set up under the Prussian police. He transferred to the Abwehr in October 1933. It was in this connection that he met future conspirators Hans Bernd Gisevius and Arthur Nebe, who were then working in the Gestapo. Oster also became a close confidant of Admiral Canaris.
Like many other army officers, Oster initially welcomed the Nazi regime, but his opinion changed after the 1934 Night of the Long Knives, in which the Schutzstaffel (SS) extrajudicially murdered many of the leaders of the rival Sturmabteilung (SA) and their political opponents, including General Kurt von Schleicher, the second to last Chancellor of the Weimar Republic, and Generalmajor (major general) Ferdinand von Bredow, former head of the Abwehr. In 1935, Oster was allowed to re-enlist in the army, but never on the General Staff. By 1938, the Blomberg–Fritsch Affair and Kristallnacht (the state-sanctioned pogrom against Jews in Germany), turned his antipathy into a hatred of Nazism and a willingness to help save Jews. In the course of the Fritsch crisis, Oster met Generaloberst (colonel general) Ludwig Beck, Chief of the General Staff, for the first time, laying the connections for the Oster Conspiracy of September 1938.
Oster's position in the Abwehr was invaluable to the conspiracy. The Abwehr could provide false papers and restricted materials, provide cover by disguising conspiratorial activities as intelligence work, link various resistance cells that were otherwise disparate, and supply intelligence to the conspirators. He also played a central role in the first military conspiracy to overthrow Hitler, which was rooted in Hitler's intention to invade Czechoslovakia. In August 1938, Beck spoke openly at a meeting of army generals in Berlin about his opposition to a war with the Western powers over Czechoslovakia. When Hitler was informed of this, he demanded and received Beck's resignation. Beck was highly respected in the army and his removal shocked the officer corps. His successor as Chief of Staff, Franz Halder, remained in touch with him and also with Oster. Privately, he said that he considered Hitler "the incarnation of evil."
Oster informed his friend Bert Sas, the Netherlands' military attaché in Berlin, more than twenty times of the exact date of the repeatedly delayed invasion of the Netherlands. Sas passed the information to his government, but was not believed. Oster calculated that his "treason" could cost the lives of 40,000 German soldiers and wrestled with his decision, but concluded that it was necessary to prevent millions of deaths that would occur in what would be undoubtedly a protracted war should Germany achieve an early victory.
Oster was arrested one day after the failed 20 July plot to assassinate Hitler. On 4 April 1945, the diaries of Admiral Canaris were discovered, and in a rage upon reading them, Hitler ordered that all current and past (such as Oster) conspirators be executed.
Fabian von Schlabrendorff, one of the few major coordinators of anti-Nazi activities to survive the war, described Oster as "a man such as God meant men to be, lucid and serene in mind, imperturbable in danger."