Officer O'Brien was killed when the patrol car he was riding in was struck by a streetcar at Tenth and Carr Streets. He had arrested a suspect, Patrick Reilly, for begging and a patrol car came to take him and prisoner to the station. The police car was traveling north on Tenth Street when it crossed the track on Carr the eastbound streetcar failed to stop at the crossing as required by law and struck the police car. The police car was driven onto the sidewalk and finally came to rest after it struck the brick wall of the Carr Street Station House. Officer O'Brien was thrown from the car and struck his head on the brick station house wall fracturing his skull. He died approximately one hour later. Everyone else in the accident received only minor injuries. The incident was ruled accidental at an inquest.

Officer O'Brien's parents were both born in Ireland. He had been with the agency for 17 years. He was survived by his wife and seven children. Interred: Calvary Cemetery, St Louis, MO.