O’Brien was born in 1837 in Ireland and moved to Alabama with his family in 1841. He joined the C.S. Navy in 1861. He served on four ships, the CSS Sumter, CSS Euphronse, CSS Alabama, and finally the CSS Shenandoah, the latter as Chief Engineer. After the Civil War he was employed as an engineer with the Morgan Steamship Line. Later he entered government service, becoming the U.S. Inspector of steam ships in Louisiana.