Osborne was found dead June 28 at his girlfriend's home. He was 55.
An autopsy report quoted the medical examiner as saying "high levels of the opiates morphine and hydrocodone" were discovered in Osborne's system at the time of his death, TMZ reported.
The medical examiner said Osborne also suffered from heart disease, which was a "significant and contributory factor" in his death.
The World Championship Wrestling offered its "deepest condolences" to the family, friends and fans of the former wrestler when he died last month.
"A rugged brawler in promotions like Mid-South Wrestling and World Championship Wrestling, Osborne made a major impact in WWE under the greasepaint of a prankster named Doink -- one of the most enduring personas of the early '90s," the WWE said in a statement.