Faced with the prospect of fighting in the war he so actively opposed, the twenty-two-year-old O’Brien felt pulled between his convictions, which could be kept intact by escaping across the border to Canada, and the expectations of those in his hometown. Though torn, he entered the military for basic training at Fort Lewis, Washington, on August 14, 1968. When he arrived in Vietnam in February 1969, he served in the Fifth Battalion of the 46th Infantry, 198th Infantry Brigade, American Division until March 1970.

O’Brien’s service brought him to the South Vietnamese village of My Lai a year after the infamous massacre of 1968. In My Lai on the morning of March 16, 1968, the United States soldiers massacred as many as 500 unarmed civilians-- old men, women, children.  O'Brien was eventually wounded and returned home with a Purple Heart, a Bronze Star for Valor, and a Combat Infantry Badge.