Home State: New York
Command Billet: Commanding Regiment
Branch of Service: Infantry
Unit: 24th New York Infantry
On the Campaign
In command of the 24th New York Infantry, part of the 1st Brigade of the 1st Division, I Corps. The Regiment was in the lead of the Division during the assault on South Mountain on September 14. Captain O'Brian was specifically commended by General Hatch for his conduct on the field.
He was wounded in action at Antietam. The Brigade advanced through the Cornfield early on the 17th in close support of BGen Gibbon's 4th 'Black Hat' Brigade.
The rest of the War
He had a leg amputated from his Antietam wound.