At the beginning of the War of the Third Coalition, O'Reilly led a cavalry division in the "Armee von Italien" of Archduke Charles, Duke of Teschen. Under his command were the "Kaiser" Chevau-léger Regiment # 1 and "Kienmayer" Hussar Regiment # 8, a total of 16 squadrons. In the Battle of Verona (1805) on 18 October 1805, Count Heinrich von Bellegarde led his corps to support Josef Philipp Vukassovich, whose division was under attack by the French. O'Reilly's division was only lightly engaged before darkness ended the fighting. At the Battle of Caldiero on 30 October, O'Reilly's division participated in the counterattacks which helped restore the Austrian line. For his actions in the war, he earned the Commander's Cross of the Military Order of Maria Theresa on 28 May 1806.