The pills could've been fake. Do you have strong proof they were legit? If you don't have a way to verify that those pills were from a trusted pharmacy, than it's easy to be duped by being sold 'pressed' fake pills that can look identical to authentic meds (same imprint, color, etc as the real deal) which means that even if you trust your source to be honest about the origins of said pills, your dealer could have been tricked into buying them unknowingly.
And of course, if you didn't even google the imprint/ color of the pill you chewed up, dealers can lie and tell you these are real. I'm assuming you knew better than that, but mentioning it just in case.
If you're certain of authenticity 100% beyond a doubt, the pills could either have been expired, or you just don't react to them as well as you hoped. I have a friend who gets fubared off of 10 mg of morphine, while I can take 10x what she does and barely notice a difference. I know morphine in particular can vary greatly in effects from user to user, and oxycodone may be the same in this sense.