O'Neill was found by his girlfriend in their Los Angeles apartment on Wednesday night. No cause of death has been determined, but O'Neill had a history of heart issues, as well as problems with drugs and alcohol, according to TMZ.
The American actor was best known for his role in Seasons 6 and 7 of FX's hit Sons of Anarchy. His character Hugh was tied to Connor (Scott Anderson), who was brokering gun deals with SAMCRO. Though Sons of Anarchy's final season had a massive body count, Hugh made it out alive.