Feldmann attended the Royal Grammar School in Bydgoszcz, the Kaiser-Wilhelm-and Ratsgymnasium Hanover, the Kadettenvoranstalt in Potsdam and the Military Academy Gross-Lichterfelde near Berlin. By 1907 he served in the German General Staff. From 1910 to 1912 he served as company commander in the 5th Grenadier Regiment, then again in the General Staff.
In 1913, Feldmann switched to Turkish service as part of the German military mission in the Ottoman Empire. There he was first a department head in the General Staff, then Chief of Staff of the First Army and finally, as Feldmann Pasha, Chief of the Operations Department in the Turkish Supreme Army Command. In this position, he participated in the Armenian genocide. Together with General Fritz Bronsart von Schellendorf, he was with the Turkish Minister of War, Enver Pasha, almost every day and coordinated with him. Feldmann once commented on this: "But it should not and must not be denied that German officers - and I am one of them myself - were forced to give their advice at certain times to free certain areas in the rear of the army from Armenians."
- Wolfgang Gust Der Völkermord an den Armeniern: Die Tragödie des ältesten Christenvolks der Welt. 1993, ISBN 3-446-17373-0, Kap. 7.
- Von Hamburger Institut für Sozialforschung, „Mittelweg 36“. 1995, S. 33.
- Deutsche Allgemeine Zeitung. vom 30. Juni 1921, Zuschrift von Otto v. Feldmann zum Prozess gegen Soghomon Tehlirian