O'Hara fought at the Siege of Barcelona in April 1706 and was wounded at the Battle of Almansa in April 1707 during the War of the Spanish Succession. He was redeployed to the Low Countries as aide-de-camp to the Duke of Marlborough and was wounded again at the Battle of Malplaquet in September 1709. He succeeded his father as colonel of the Royal Fusiliers in January 1713 and then served with his regiment in Minorca for several years. For this he was appointed an aide-de-camp to the King in 1717 and, while serving in Ireland, was created Baron Kilmaine in the Peerage of Ireland on 2 January 1722. He succeeded his father as 2nd Baron Tyrawley in June 1724.