Hmmmmmm.....I dont think so. I think you would need a VERY big tank to keep both pirhana and oscar.You would also need a very large filter as they are both very messy fish,even then large,frequent water changes.
Even if you could do all that I still would not chance it,I have been told that pirhana must be kept in a species tank,however others may say diffrent.........
If your tank is big enough,why not get a black belly pirhana or a golden one.