Before 1919: In the militaryFrom 1889 to 1895, Oehler was a student at Pforta. He then joined the Prussian Army, although he also showed considerable interest in literature and especially music. He would often express to his cousin Elisabeth Förster-Nietzsche a wish to work at her "Nietzsche-Archiv" someday. Nevertheless, his main interest was his career in the army, whose values he deeply admired. He was made Oberleutnant in 1908, Hauptmann in 1912, and was allowed to call himself Major when he left the army in 1919. For a long time, he was stationed in Deutsch Eylau, today Iława.
He was granted leave from April 1 till December 31, 1908, to work at the Nietzsche-Archiv in Weimar. In the summer, he travelled to Ernest Thiel in Stockholm as Elisabeth Förster-Nietzsche's emissary. Thiel, a banker and patron, had agreed to give a considerable amount of money for the founding of Förster-Nietzsche's "Stiftung Nietzsche-Archiv" (Nietzsche-Archiv foundation), despite his being partly of Jewish ancestry. (Förster-Nietzsche had married Bernhard Förster, an antisemitic agitator, but by this time had renounced antisemitism). At the end of 1908, Oehler became a member of the new Stiftung's Vorstand, or managerial board, on which he remained until 1945.
Oehler took part in the Battle of Tannenberg (1914) early in World War I, but soon became unable to fight due to a case of sciatica. He performed bureaucratic activities for the remainder of the war, first in Marienburg (Malbork), and then at the Ministry of War in Berlin. He had great faith in the possibility of a German victory and was shocked by the German November Revolution. In the months following the war, Oehler stayed on at the Ministry of War and, together with other officers, tried to regain power from the revolutionary Soldatenräte (soldiers' councils, see workers' council), whom he despised.