[2] Ostermann was of such short height that wooden blocks had to be attached to his rudder pedals for him to engage in tight turning aerial combat.

Ostermann joined the Luftwaffe as a Fahnenjunker (Officer Cadet) in March 1937 after he had received his Abitur (diploma)—the final exams that pupils take at the end of their secondary education.

His first assignment was with I. Gruppe (1st group) Zerstörergeschwader 1 (ZG 1) flying the Messerschmitt Bf 110 and participated in the Invasion of Poland in 1939.

JG 21 at the time was based at Mönchengladbach and was subordinated to Oberstleutnant (Lieutenant Colonel) Max Ibel, the Geschwaderkommodore (Wing Commander) of Jagdgeschwader 27.

[3] Ostermann was appointed the commander of a Rotte, an element of two aircraft, with Unteroffizier (non-commissioned officer) Fritz Marcks as his wingman.

In a head on firing pass two 20mm shells tore off large parts of the aircraft's tail fin, which then collided with Osterrmann's starboard wing.

[9] On 5 September 1940, III./JG 54's Gruppenkommandeur (group commander) failed to return from a mission and Geschwaderkommodore Hannes Trautloft temporarily appointed Oberleutnant Günther Scholz to lead the Gruppe.

He radioed his fellow pilots: "Spinat vier meldet sich ab nach Kanada—Spinach 4 reports off for Canada".

Five month later following a long combat pause, JG 54 was moved to the south-east to counter the pro-British coup d'état in Belgrade, Yugoslavia.

[12] He claimed his ninth victory over a Yugoslav Royal Air Force Messerschmitt Bf 109E-3, piloted by Karel Štrbenk who was killed, on 6 April 1941 over Belgrade during the Balkans Campaign.

[13][14] On 23 June 1941, during the opening phase of Operation Barbarossa, the German invasion of the Soviet Union, Ostermann, in a free-hunting mission in the Lithuanian airspace north of Kaunas, intercepted a formation of nine Tupolev SB's and claimed two shot down in flames.

A German police officer had assumed that Ostermann, with his childlike features, was actually a schoolboy who was playing a prank and illegally wearing a Luftwaffe uniform and military decorations.

[2][25] Ostermann claimed his 100th aerial victory on 12 May 1942, the second JG 54 pilot—Hans Philipp was the first—and sixth overall to achieve the century, though in the same engagement his Bf 109F-4 was hit and damaged.

He and his wingman Unteroffizier Heinrich Bosin were flying at an altitude of 1,000 metres (3,300 ft) when they spotted a formation of nine Curtiss P-40 Warhawk's.

Ostermann's Bf 109G-2 (Werknummer 10438—factory number) was hit in the cockpit by 41 IAP's (41st Fighter Aviation Regiment) Starshiy Leytenant (First Lieutenant) Arkady Ivanovich Sukov flying a LaGG-3.

This made him the first Swords recipient to be lost in air combat, as Mölders death was accidental and Steinbatz had only received the Oak Leaves before he died.