Civil War Union Army Officer. An 1854 graduate of the United States Military Academy at West Point, New York, his classmates included future Civil War Union generals Oliver Otis Howard, Thomas H. Ruger and Stephen H. Weed, as well as future Confederate generals J.E.B. Stuart, Stephen D. Lee and George W. C. Lee. Posted to the Indian Territory (the future state of Oklahoma), he spent the next four years on the western frontier, serving at one point as the Regimental Quartermaster for the 7th Infantry regiment before he resigned his commission in October 1858. He then resided in Beloit, Wisconsin, where he became a lawyer and lumber merchant. After the Civil War began he offered his services to preserve the Union, and was commissioned Colonel and commander of the 2nd Wisconsin Volunteer Infantry on August 2, 1861. The beginning of his tenure was rocky, as the regiment had fought at the 1st Battle of Bull Run in July 1861 under a different officer who resigned, and Colonel O’Connor being an outspoken Democrat alienated many men of his new command. However, he would win his troops over with his military discipline and leadership. The regiment was brigaded with three other Wisconsin units and an Indiana unit to form what would become know as the “Iron Brigade” under the leadership of Brigadier General John Gibbon. Colonel O’Conner led his men through operations in Virginia in the spring and summer of 1862, but the regiment did not see any action until the August 1862 Second Bull Run Campaign. In August 28, 1862 he directed his unit as it fought at the Battle of Brawner’s Farm, near Gainesville, Virginia; an engagement that was a precursor to the looming larger battle at Bull Run. During the battle the Iron Brigade fought toe-to-toe in an open field against Confederates anchored by the famed “Stonewall Brigade”, with both sides inflicting heavy casualties on each other and both sides solidifying a reputation for bravery. During the fight Colonel O’Conner place himself on his horse behind the regiment colors, and was shot down twice, with the second wound being mortal, dying within an hour being hit. His remains were eventually interred in Arlington National Cemetery, Arlington, Virginia.