"Oxymorphone(OPANA) is also produced within the human body where the liver metabolises oxycodone by means of O demethylation catalysed by the CytochroEREme P-450 enzyme family, in particular the II-D-6 and III-A-2 thereof. Approximately 10 per cent of the dose is processed by the endocrine system in this respect; this can vary widely from person to person. The codeine-hydrocodone group and morphinans exhibit the same characteristics."
I dont know to much about oxymorphone(Opana) but this little stub from wikipedia says(or atleast I think it says) that this drug is metabolised exactly the same way as oxycodone therefore its like if you took vicodin and then a few days later took heroin you wouldnt have a tolerance to the heroin because you took vicodin a few days before and not heroin. but with oxymorphone(opana) there is a cross tolerance between Opana and oxycodone. not like say morphine and oxycotton. although morphine and codeine also have a cross tolerance just like opana and oxycotton because codeine gets metabolised into morphine. so its pretty much as if you were taking oxycodone or when you were taking the Opana, basically Opana is just a stronger version of oxycotton so no wonder u built a tolerance, Opana is like two times stronger than oxycodone but their basically the same except for the stronger potency of opana. im not sure if this is true but this would be my guess.
oh and inccase you dont know oxycodone and oxycotton are the same drug, just incase u got confused from that