O'Brien earned his BA in 1968 in political science from Macalester College, where he was student body president. That same year he was drafted into the United States Army and was sent to Vietnam, where he served from 1969 to 1970 in 3rd Platoon, Company A, 5th Battalion, 46th Infantry Regiment, part of the 23rd Infantry Division (the Americal Division) that contained the unit that perpetrated the My Lai Massacre the year before his arrival. O'Brien has said that when his unit got to the area around My Lai (referred to as "Pinkville" by the U.S. forces), "we all wondered why the place was so hostile. We did not know there had been a massacre there a year earlier. The news about that only came out later, while we were there, and then we knew."