I can confirm what neighborhoodthreat says,
yeah OP, chances are you are one of the 'lucky' (or not depending how you look at it) ones who metabolizes more codeine into active opiates than most.
I can't exactly remember now what my codeine dose was at the time, something around 500mg I believe (it should be documented here on BL somewhere).. I got a hold of some oxycodone just like you. 500mg codeine didn't really get me that high at this point in time, its just the dose i took each day that i could afford to maintain at that point. I felt good, but like you.. 120mg used to get me wrecked to the point of a period of nodding right at the peak. Or maybe I was up to 160-180 by that point. Those early days are so blurry.
I knew I'd have some tolerance, but at this point presumed I was 'normal' on the codeine metabolisation scale and that friends I'd introduced just happened to be the few that barely metabolize any. Turns out they were probably just normal. Anyhow
First I tried 40mg oxy. (IR. no time release mechanism involved).
Pretty much nothing.
Then I tried 60mg. I felt it, but not as much as my usual codeine dose.
Then I tried 80mg and it got me to about the same place as my codeine dose did. Not quite there though.
I tried some combos too, eg 20mg oxy on top of my codeine dose, etc... I think the oxy affects metabolisation of codeine into morphine/c-6-g though, so it wasn't particularly productive. I played with the timing but didnt have enough left to figure out the best way to do it.
At this point I ran out, and did not have a continuous supply. Which is for the best, because there's no way I could afford 100+mg oxy a day.