how much oxycodone, oxymorphone and hydrocodone needs to be ingested to cause death?

My dad died two months ago, the dr.'s said it was of natural causes but when I got the death certificate, it said the immediate cause of death was overdose of oxycodone, oxymorphone, and hydrocodone, but it also said it was accident. I'm thinking it says only an accident because there wasn't a suicide letter or anything. Any help?

: http://alcoholdrugstreatment.info/how-much-oxycodone-oxymorphone-and-hydrocodone-needs-to-be-ingested-to-cause-death