Osterkamp was born in Rölsdorf near
Düren, West Germany, and grew up in
Aschersleben, modern day
Saxony-Anhalt. He was born in 1892 as the second son of the factory owner Hermann Osterkamp and Anna Wilhelmine née Blank. Osterkamp received his
Abitur from the
Gymnasium in
Dessau. His schoolmates in Dessau included the future pilots
Oswald Boelcke and
Gotthard Sachsenberg. When the First World War started he was studying
forestry but decided to enlist in the German Army. He was rejected for service due to his "slight build" and he instead enlisted in the
Marinefliegerkorps in August 1914. He then flew with the
2. Marine-Fliegerabteilung in
Flanders. 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. . ==Interwar years==