Cimetidine will do the same thing (pretty much) as grapefruit juice, only it's more effective. It's OTC in some countries, RX only in others. Other methods are more synergistic than potentiators. Diphenhydramine (Benedryl) causes drowsiness and can synergize to increase the nod from oxycodone. Dimenhydrinate (Gravol) likewise. Benzodiazapines will do much the same as alcohol. Basically most things people call "potentiators" for opiates are CNS depressants and other drugs that make you drowsy. They don't really potentiate... One benefit of a low dose of diphenhydramine is that it can help with the itch associated with opiates. Dimenhydrinate likewise can help with any nausea. I have used both for the itch and nausea respectively, but in my opinion, anything that makes you drowsy takes away from the more stimulating high of oxycodone. YMMV.
Edit: Not to contradict Effie, but you will want to use that chart with a whole lot of caution. While there are plenty of drugs there that will inhibit the various CYP450 substrate enzymes, there are also plenty of drugs there that could kill you. For example I would highly discourage anyone using Warfarin to potentiate an opiate high.
The only common side effect of warfarin is hemorrhage (bleeding). The risk of severe bleeding is small but definite (a median annual rate of 0.9 to 2.7% has been reported) and any benefit needs to outweigh this risk when warfarin is considered as a therapeutic measure. Risk of bleeding is augmented if the INR is out of range (due to accidental or deliberate overdose or due to interactions), and may cause hemoptysis (coughing up blood), excessive bruising, bleeding from nose or gums, or blood in urine or stool.
The risks of bleeding is increased when warfarin is combined with antiplatelet drugs such as clopidogrel, aspirin, or other nonsteroidal anti-inflammatory drugs. The risk may also be increased in elderly patients and in patients on hemodialysis.