Colonel O’Connor was born on August 29, 1833, at Cleveland, Ohio. At the age of 16, he received an appointment to the United States Military Academy at West Point. He graduated in 1854 and was commissioned a 2nd Lieutenant of Infantry.

He served in the “Indian Territory” until October 22, 1858, when he resigned his commission and became a resident of Beloit, Wisconsin, with his wife Jennette, whom he had married on September 1, 1857. Colonel O’Connor and his wife were the parents of two daughters, both of whom died in infancy.

Colonel O’Connor practiced law in Beloit until the commencement of the Civil War. Answering the call of President Lincoln for volunteers, he applied to the Department of the Army for reinstatement of his regular Army commission. Colonel O’Connor made this decision even though, like many graduates of West Point, he had relatives, friends, and property in the rebel states and had been in favor of compromise prior to the shelling of Fort Sumter.

Before the federal government could reinstate his commission, Colonel Randall, smarting from criticism of his appointment of political incompetents to lead the 2nd Wisconsin Infantry Regiment, chose Edgar O’Connor because of his military experience as colonel and commanding officer of the 2nd Wisconsin, which became part of the “Iron Brigade.”

On August 28, 1862, Colonel O’Connor was part of the northern army moving north to defend Washington. Near the site of the Battles of Bull Run, a Confederate artillery battery was seen moving into position on a small hill near a farmhouse and barn. The 2nd Wisconsin was the first regiment in the marching column and was ordered by Brigadier General John Gibbon to drive off the Confederates.

Unknown to Colonel O’Connor and General Gibbon, the Union Army had stumbled upon a Confederate Army of 25,000 men hidden behind the hill. Colonel O’Connor formed a line and, with the 2nd Wisconsin Infantry Regiment, began marching up the hill. As the 430 men of the 2nd Wisconsin approached the hill, they saw rising before them over the crest two lines of 4,000 Confederate infantrymen marching toward them.

The 2nd went into the fight at the Battle of Brawner Farm with 430 men, and when the fighting was done, the 2nd had 56 killed, 205 wounded, and 30 missing. Colonel O’Connor was shot from his horse with wounds in the arm and groin and died within the hour.

Colonel O’Connor is buried in Arlington National Cemetery. His grave is marked by a five-foot polished marble monument erected by the survivors of the “Iron Brigade.”