Trans-Siberian Orchestra's Paul O'Neill Died from Prescription Drug Overdose

Paul O’Neill — the founder of the Trans-Siberian Orchestra — died from a combination of prescription drugs … TMZ has learned.

TMZ obtained a copy of O’Neill’s autopsy report, and it says he died of intoxication from a mix of methadone, codeine, diazepam (generic Valium) and doxylamine (an antihistamine).

According to the doc, the cause of death is intoxication … but it also mentions other ailments O’Neill was battling … such as mild heart disease, hypertension and moderate hardening of the arteries. The medical examiner determined the death to be accidental.

When O’Neill died last month, the band released a statement saying he’d died from a “chronic illness.”

Paul was 61.