The potential problem with opiate based painkillers (and Oxycontin i.e. Oxycodone, Hydrocodone i.e. Vicodin, both apply here) is, as BilZ0r correctly said above, is that the same enzyme (CYP2D6) in the liver/renal system metabolizes both *codones and MDxx. The risk is that the level of MDMA (or opiates) in your system will remain too high for too long.
If you have "a bit of an Oxy habit" (I know what I you mean, I used to chip Oxy sometimes) then you probably know about potentiators like Tagament, white grapefruit juice and etc. You're bascially doing the same thing.
If your usual Oxy dose is... say... 20mg or 40mg (and do you snort? chew?) I would drop about 33% off it. I would also start with a half an MDMA just to be safe and drop the other half after you're sure you're ok.
It would help to know more about your Oxy use and tolerance. I never got much above 100mg in a day but I know quite a bit about about Oxy and its effects so I'd be happy to give you more specific suggestions.
OTC painkillers (aka APAP, aka Tylenol, Asprin, etc.) are a bad thing to be taking when your taxing your system. Tylenol works because it breaks down... oh... just barely before it starts doing liver damage. You don't really want to be taking it when you have reduced renal/liver capacity or your on anything hard.
That's very general advice but people as a whole VASTLY underestimate the long-term cumulative damage acetominphin can do. The biggest problem is that the damage doesn't usually show for months.
My advice: Steer clear of it. Asprin if you really must.