Private, Company B, 7th US Cavalry Regiment. He was on detached service at the Yellowstone Depot at the time of the Battle of the Little Big Horn. Had he been at the battle, it is likely he would have survived, since his company had been assigned to protect the packtrain, and missed most of the actual battle.
He enlisted on March 31, 1876, in Boston, giving his previous occupation as Currier. He had blue eyes, light hair, fair complexion and was 5 feet 5 1/2 inches tall. From the official records of L.A. McKinney, Asst. Surgeon on duty at Fort Buford: "Thomas O'Brien died 4 1/2 am of typhoid fever, complicated by pneumonia of the lower lobe of the right lung. The man came into the hospital complaining of chills and ague and diarrhea. He had considerable fever, but temperatures never exceeded 100 degrees at any time, and the thermometers are normal."
He was initially buried in the Post Cemetery at Fort Buford, and was reinterred in the Custer Battlefield National Cemetery.