Immigrated to Pennsylvania (1772); worked as an Indian trader and invested in western lands. Served in Revolutionary War as an army officer, hospital commissary (1781), and assistant quartermaster to General Nathanael Greene (1781-83). Settled in Pittsburgh as a merchant (c. 1783); received many government contracts to provision troops in the region. Quartermaster of the U.S. army (1792-96). Helped establish the Pittsburgh Glassworks (1795). Pioneer in salt and cotton trades. President of the Pittsburgh branch of the Bank of Pennsylvania (1815). Federalist.
Son of John O’Hara of County Mayo, Ireland. Educated at the Jesuit College of Saint Sulpice in Paris. Married Mary Carson (1783); six children.