Lieutenant Colonel O'Brien's official Medal of Honor citation reads: For conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty at Saipan, Marianas Islands, from 20 June through 7 July 1944.

With bloody hand-to-hand fighting in progress everywhere, their forward positions were finally overrun by the sheer weight of the enemy numbers.

Striding up and down the lines, he fired at the enemy with a pistol in each hand and his presence there bolstered the spirits of the men, encouraged them in their fight and sustained them in their heroic stand.

Even after he was seriously wounded, Lt. Col. O'Brien refused to be evacuated and after his pistol ammunition was exhausted, he manned a .50 caliber machine gun, mounted on a jeep, and continued firing.

[1]The U.S. Army ship USAT Col. William J. O'Brien, which served in the Pacific Ocean at the end of World War II, was named in his honor.

In November 2009, a memorial honoring O'Brien and fellow Troy natives Major General Ogden J. Ross and Sergeant Thomas Baker was installed in the Rensselaer County office building.