After he received the Knight's Cross of the Iron Cross with Oak Leaves () for 62 aerial victories on 12 March 1942 he was appointed "Staffelkapitän" (squadron leader) of the 8./JG 54 (8th squadron). His 80th and 81st aerial victory were claimed on 19 March 1942 followed by victories number 89 and 90 on 27 April 1942. 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. Ostermann himself was hit in the right arm and upper thigh. Although severely wounded, he managed to return to his home airfield. Five days later, while in the hospital, he was awarded the Knight's Cross of the Iron Cross with Oak Leaves and Swords (). The award was presented at the "Führerhauptquartier", the "Wolf's Lair" () at Rastenburg, on 28 and 29 June 1942.