Hi welcome to Bluelight! I think it's pretty much the same as taking orally. If you are determined to plug, make sure you clear your bowels. Otherwise, it may not be absorbed properly. Really, taking oxycodone orally is the best ROA. (moving to BDD)
I'd like to extend the welcome! ^ T. Calderone is correct, the only real benefit plugging will give is a slightly shorter onset of effects and doesn't suffer from first pass metabolism or the contents of your stomach. As far as bioavailability goes oral and rectal are probably on par.