It depends on what you're looking for and it would help to know if you've taken either Oxy or shrooms seperately before.
Either the Oxy or the shrooms could make you slightly nauseas. Taking them together could increase the nausea. If you know you've got an iron stomach or that neither the Oxy nor shrooms have ever made you nauseas, then you're okay.
If you want to reduce the nausea, take one or the other 20-30 minutes before the other.
If it was me, I'd take them both at the same time. That way they'd hit at the same time and have me that more messed up, which I always enjoyed when I was combining other drugs with psychedelics.
If you want to experience the mushrooms alone, then take the Oxy a little while after the shrooms.
Or if you want the Oxy to help set the mood of the shrooms by making you more euphoric and putting you in a good mood before you even start tripping, take it 20-30 minutes before the shrooms.
No matter what you do, it shouldn't make a huge difference.