In late June 1916 he joined the 16th Battalion's newly formed Scouting Section in northern France and served as a scout, observer and sniper during his time on the Western Front in Belgium and France.
Between 9 and 12 August 1916 at Mouquet Farm, Pozières, during four days of very heavy fighting, Private O'Meara repeatedly went out and brought in wounded officers and men from "no man's land" under intense artillery and machine-gun fire. He also volunteered and carried up ammunition and bombs through a heavy barrage to a portion of the trenches which was being heavily shelled at the time.
O'Meara was wounded three times during the war: near Mouquet farm in August 1916, near Bullecourt in April 1917 and near Messines in August 1917.
He was presented with his Victoria Cross medal by King George V at Buckingham Palace on 21 July 1917.