Oxycodone and MDMA are both primarily broken down by the same enzymes (CYP 2D6), so mixing the two could dangerously potentiate both.
Since MDMA itself tends to suppress the sex drive, it seems unlikely that it will help you overcome a lack of libido caused by opiates. Methamphetamine would probably be a better bet, although that too could be dangerous.
If you're determined to mix amphetamines (like MDMA) with oxy, start with low doses.
Better yet, try to kick the oxy habit if you don't need it to control physical pain.