O'Reilly was promoted to General-Major during the War of the First Coalition in 1794. He achieved distinction at the Battle of Amberg on 24 August 1796. At the First Battle of Zürich on 4 June 1799, he led a brigade in Friedrich Freiherr von Hotze's Left Wing. His command consisted of one battalion of the "Peterwardeiner" Grenz Infantry Regiment # 9, three battalions of the "Kaunitz" Infantry Regiment # 20, two squadrons of the "Coburg" Dragoon Regiment # 6, and two squadrons of Grenz Hussars. He transferred to the army of Michael von Melas in Italy and received promotion to Feldmarschallleutnant on 6 March 1800.