You should be aware that there are dfefinitely statistical outliers regarding the objective effects of Opioids. It's exremely rare, but there are people out there who for one reason another are completely immune to the effects of exogenous Opioids. They believe it has something to do with a gene that creates (I think?) Opioid receptors. As I said, it's extremely rare, but I just want to make the point that Opioids are like any other drug; there's a wide variability between objective and subjective effects.
I, too am relatively unaffected by Oxycodone. I can take 5 grams of Codeine (it can be quite cheap in certain places) to take away my withdrawal. On the other hand, Morphine/Heroin and Hydromorphone (Dilaudid) seem to effect me in a relatively normal manner, although I would still say that my baseline, innate tolerance is about 3 times that of the average user. I've never been knocked out by a single bag. I've never overdosed. I have no fear doing 4 bags out of a fresh batch. This is not responsible behavior, but I'm just trying to give you an accurate illustration of what it can be like.
I've read case reports of folks who require hundreds of milligrams of Hydromorphone today to control their pain due to the aforementioned phenomena. It's not totally unheard of or even strange that Oxycodone has a perceptibly less potent effect upon you than other Opioids. It would appear that your unique physiology ranging from the enzymes that metabolize drugs to the Opioid receptors themselves, makes Oxycodone less effective for you.