If anything that Oxy is going to dull your come up and possibly even your peak. This also depends on your tolerance and all but like an above poser stated, save the Oxy for the comedown.
Never said it was 'bad'. It is what it is. I enjoy mixing opiates with various stimulants also, but MDMA has a pretty rounded profile that I wouldn't care to mellow out in any way compared to say.. crack or PV's. Usually when I roll, the idea isn't to be relaxed and sedated. And if that was the case I'd probably use 4-EMC.