Sergeant Onstott was a member of Company A, 1st Battalion, 5th Cavalry Regiment, 1st Cavalry Division. He was listed as Missing in Action while fighting the enemy in South Korea on 25 July 1950. He was presumed dead on 19 January 1954. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Onstott was awarded the Purple Heart, the Combat Infantryman's Badge, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.