Sure, no one says you cant mix. Just dont overdo it. Both have slighty different highs as they stimulate different combos of opiate receptors.
-Oxycodone- Very euphoric, at least while your tolerance is low.
-Morphine- More of a pain killing stone. (analgesic)
So go ahead and mix and see what happens. They will multiply on each other though so be careful. Maybe one morphine and 30mg of oxy or so?