John Thomond O'Brien (1786–1861), was an Army officer born in 1786 in Baltinglass, Co. Wicklow, son of Martin O'Brien and Honoria O'Connor.
Career John Thomond O'Brien left Ireland and arrived in Buenos Aires in 1812 to open a merchant house. He enrolled in the army and fought in Uruguay with General Soler. In 1816 O'Brien joined José de San Martín's mounted grenadiers regiment in the Andes army. After the battle of Chacabuco, he was promoted to captain and appointed aide-de-camp to general San Martín. O'Brien fought in the Second Battle of Cancha Rayada and at Battle of Maipú. In 1821 he was promoted to colonel, and died in 1861.