Absorption does not always directly correlate with how the drug is experienced. Obviously, when you IV heroin, you're going to get a stronger high than when you insufflated it as you have pretty much 100% absorption (and heroin has good legs). With other drugs and ROA, it can be more tricky. My experience with oxy is limited, so I'll use methadone as an example as it also has a high oral BA. I've found that rectal administration of methadone, (though it has a lower BA than oral) hits me way harder, and lasts just about as long. The reason, I theorize, is due to the much quicker onset and peak. I can actually cut my dose down by 25% and get the same effect I would, had I taken it orally.
Another example is how absorption is offten increased when taken with a fatty meal. While the numbers might be there, it's been mine (and many other's experience) that it does not increase the effects, but rather diminishes them.
With oxycodone I imagine it's similar, though I've heard the duration is shortened pretty significantly. The best ROA is really oral as it has the highest BA next to IV, but lasts way longer.
