He fought in the Chilean War of Independence. John Thomond O'Brien left Ireland and arrived in Buenos Aires in 1812 to open a merchant house. On his way there, his ship crashed onto the rocks of Fernando Po Island in west Africa.
Only he and a few others survived to continue their journey. Soon after arriving in Buenos Aires, the local people gave him the name Don Juan O'Brien. He enlisted 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 in Lisbon, Portugal while returning to South America.