After Osthoff had taken part in the First World War from 1915 to 1918, he studied musicology, history of art and philosophy from 1919, first at Westfälische Wilhelms-Universität in Münster and from 1920 at Humboldt-Universität zu Berlin.
[1] In 1938, as a participant in the musicological conference within the framework of the Reichsmusiktage, Osthoff gave a lecture on the topic Das Besetzungsproblem in der Musik des Barockzeitalters.
[4] However, this project broke down with the beginning of World War II, in which Osthoff participated in the Battle of France as a lieutenant (reserve officer) of the Wehrmacht until 1940.
[5] Osthoff concealed his activities in Belgium in his self-portrayal in Die Musik in Geschichte und Gegenwart and wrote only: "In 1939/40 he was a war veteran.
[6] In the winter semester of 1940/41 Osthoff resumed his teaching activities at Frankfurt University, but remained a staff member in the main music department of the Fuehrer's commissioner for the supervision of the entire intellectual and ideological training and education of the NSDAP.