I am buying a new tiger Oscar and I'm wondering if it will live with a Pacu fish or piranah:

What sort of piranha? If it's a meat-eating kind, don't bother. They are best kept with their own. As for pacu, yes you can keep them with Oscars, however you need an extremely large tank because pacu can grow to 3 feet in length.

red bellies

Absolutely not. Most people don't know a lot about piranha and they think that if you put your hand in the tank, they will shred it down to the bone in seconds. This is not how piranha feed. In fact, if you had a very large tank, you could quite easily go swimming in it and come to no harm at all. However, if you had an open wound, you would probably be in trouble. Piranha will pick on week fish mainly, they will feed off carrion that has fallen into the river. They don't normally go after healthy creatures. The problem is, putting your Oscar in with piranha in a small environment would be a very risky thing to do. If your Oscar was to become ill and not its normal self, the piranha would probably eat it straight away. So I would avoid mixing the two types of fish.

Thanks for that mate, you are awesome.

I would still like to get some red bellies, but then I would have to keep my water clean and my fish happy and healthy.
FRUSTRATED!
Might just get Oscars.

Whereabouts do live? Some states in America prohibit the keeping of piranha.

Stick to one or the other, or get two tanks

I live in England mate, still love my fish though. Yeah i'll stick with your suggestions.

Piranhas are quite difficult to keep. Feed them too much and your water quality suffers, dont feed them enough and they start to take chunks out of each other. I have never kept them but have seen some in shops with bits missing and it amazing how big a chunk even a 4 inch one can take

Oscars live with piranah in the wild, and have been seen grouping together and protecting a hurt one against a school of red bellies. Many say they make great tankmates....at least, unless the oscar gets sick or old...then the oscar will be oscar no more as OFL mentioned. It would certainly work in a tank of 240 gallons or more with several oscars and several piranahs.

If you plan on keeping them seperately which i would advise, you will need several piranha to start out with. I recommend no more then five or six. Keep in mine they can reach ten inches or more in length, and have voracious appetites. If they are underfed they will not hesitate to bite the hand that feeds them. You will need an apt filteration system as they are very messy feeders(double filteration recommended) and a steady supply of live food. They are most certainly not beginner fish and i would only recommend them if you have had several years of expierance. Another option as opposed to red bellies would be silver dollars, which look similar in apperance and make better tankmates for oscars. There is a great article in the members articles section regarding silver dollars:)

Cheers,
Art