As commander of the Fire Zouaves, he rallied around 150 infantry against approaching rioters in front of Oliver's Livery Stable near the East River.
After a brief skirmish with the rioters, the mob retreated and O'Brien left his command and walked up the avenue entering a nearby drugstore.
Severely beaten by the crowd, he was kicked and hit with stones as he lay on the street, which continued for more than an hour.
He was eventually taken by rioters to his nearby home where he was tortured to death and mutilated beyond recognition.
After rioters had left, O'Brien's body was transferred to Bellevue Hospital and subsequently buried in a pauper's grave at Calvary Cemetery.