 During 1915–1916, he served as an air observer, and became the first German pilot to fly a land-based aircraft to England on a reconnaissance-mission.Osterkamp claimed his first (but unconfirmed) kill on 6 September 1916 as an observer to pilot "Leutnant zur See" Wilhelm Mattheus in a LVG C.II two-seater aircraft.In March 1917, he joined the "Kampffliegerschule" (Combat pilot school) in Putzig and then joined "Marine Feld Jagdstaffel 1" on the 14th of April 1917 On 21 March 1917, "Leutnant" Osterkamp took command of "Marine Feld Jagdstaffel 2" He scored a total of 32 victories during the war, and was awarded the Prussian military order "Pour le Mérite" on 2 September 1918, and was one of the last individuals to receive it.