O'Meara's first operational posting was to No. 64 Squadron based at RAF Hornchurch, with which he obtained his first 'kill' while over Dunkirk on 31 May 1940 when he brought down a Bf 109. He had already damaged a Ju 88 off Calais on 21 May. His next claim was a Bf 109 of JG 51, shot down in flames over the English Channel on 19 July and ten days later, while intercepting a raid over Dover, claimed two Ju 87s. On 11 August he claimed 2 Bf 109 'probables' and on the following day destroyed one more. He claimed a Bf 109 down on 13 August, and on the 15th he damaged three Heinkel He 111 bombers. On 18 August O'Meara claimed shared destruction of a Ju 88 and a He 111 destroyed.