On 18 August 1940, III./JG 51 escorted Dorniers attacking Hornchurch airfield. Intercepted by Hurricanes over Kent, Oesau claimed one Hurricane shot down as his 20th victory, earning him the Knight's Cross of the Iron Cross () two days later, the first pilot of JG 51 to be thus honored. On 24 August 1940, Trautloft took over as "Geschwaderkommodore" of "Jagdgeschwader" 54, and Oesau replaced him as "Gruppenkommandeur" of III./JG 51. By October 1940, Oesau had a total score of 48 (including 26 Spitfires).