He fought for France against Spain in 1718 with the rank of colonel in the service of his father's regiment and later fought in the War of the Polish Succession in the Siege of Philippsburg in June 1734, where he was wounded.
O'Brien also fought in the Battle of Dettingen in 1743 and in the Battle of Fontenoy in 1745.
O'Brien was invested as a Knight, L'Ordre du Saint-Esprit of France on 2 February 1746 at the chapel of Versailles, ĂŽle-de-France, France.
He held the office of Governor of Neuf-Brisach in Alsace, and was Commander-in-Chief of the province of Languedoc.