Only a small portion of oxycodone is metabolized to oxymorphone by cytochrome P450 2D6 enzyme, and the rate at which that takes place depends on how active the enzymes in your liver happen to be (genetic differences lead to different rates in different people).
It's not obvious at all how much the oxymorphone metabolite takes part to the effects of oxycodone. Oxycodone itself has opioid effects, unlike codeine which has to be metabolized to morphine and glucuronide derivatives before it takes effect.