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. Ostermann survived a belly landing following combat with more SBs in south-eastern Latvia on 26 June. On 5 July he claimed three SB-3 Soviet bombers in combat over the Velikaya River at Ostrov. He claimed his 19th and 20th aerial victory on 6 July in the same combat area. On the Eastern Front he claimed JG 54's 1,000th victory of the war on 1 August 1941. He became the eighth member of JG 54 to receive the Knight's Cross of the Iron Cross () on 4 September 1941 after 29 aerial victories. The award was presented on 10 September 1941 in Dno by "Generaloberst" (Colonel General) Alfred Keller. He claimed his 50th aerial victory on 9 January 1942, the 60th on 28 January 1942 and the 70th on 19 February 1942. After this series of aerial victories Ostermann was sent on home leave. The reason for his leave was that he wanted to get married. Back home, on his way to his wedding ceremony, Ostermann was arrested and put in jail. 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. The consequences endured by the police officer for his bad judgement remain unknown.