EXCLUSIVE

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."