IMO, opiates (IV heroin aside, although I recommend against using heroin, especially IV) are kind of difficult to combine with other drugs, and adderall in particular, doesn't play nice with other drugs, particularly opiates (even more particularly oxy, just like in this context), and the combo IME creates dysphoria if I dose anything higher than just above threshold with either of them (however, small doses of each is great in social situations), as not only does the oxycodone get overpowered by the adderall, but the adderall gets dulled by the oxycodone, and I believe that l-amphetamine is the reason for this, because d-amphetamine and oxycodone is a decent combo, and they don't seem to overpower each other either, however they don't synergize, but they combine fairly well.
IME, the only drugs that synergize with opiates like oxycodone or hydrocodone are DRIs. By DRIs I mean cocaine, ketamine and methylphenidate (the only way to enjoy MPH IMO is with opiates). I don't know why it is, seeing as cocaine/MPH are stimulants, and ketamine is a dissociative, but for some reason, DRI drugs seem to generally synergize with opiates very well. Even cannabis seems to ruin the opiate high for me.
IMO the opiate high is very fragile and subtle, and I typically don't mix it with anything other than nicotine, however many people seem to enjoy combining opiates with other drugs.
HOWEVER, with IV morphine-based opiates (which is to be avoided at all costs), particularly heroin, I find that it combines pretty well with almost all other drugs, even the dirtier drugs like adderall (or racemic methamphetamine), and alcohol.
NONE OF THIS MEANS THAT COMBINING DRUGS IS SAFE, COMBINING ANY OPIATE WITH ANY CNS STIMULANT OR DEPRESSANT IS DANGEROUS, AND IS TO BE PRACTICED WITH GREAT CAUTION. DOSE LOW AND TITRATE UP FROM PRESCRIBED (or commonly accepted as safe) DOSE.