i added oscars to my piranha tank for a lil while. maybe 2 months then i removed them. the oscars just looked stressed like hell everyday. they always got picked on and i felt bad for them. if you dont need to do it, i wouldn't. i did it as an emergency kinda thing, i had no tank for them at that moment. but if you wanna try it feel free, that sulking that the amazonian is talking about is so retarted but true, oscars sit sideways like there drunk or dead. the piranhas can and most likely will strike at the oscar when he does this. i got lucky and mine "sulked" behind some rocks for a few days b4 showing his face.