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.