If we are talking about the characin ive had that mix before and it has worked. Just remember that the oscar will grow quicker than tyhe flatil so it could eventually become food. Just make sure that the flagtail has a bit more size and a headstart over the oscar. But these are very territorial and you cannot keep 2 to a tank single specimins only really
Those are the ones im talking about. Ive kept a few and worked with them in the past and currently have one around 9" had him for quite a while now got him in august 07 if memory serves. Great fish. i love em
theres a really good chance the Prochilodus will beat the crap out of eachother.. they dont do well in groups of 2..
they may also decide that they like slime coat at somepoint which would be bad for the O
Bogwoodbruce is definately right about giving the flagtail a headstart in terms of size. This way it can make an impression on the oscar a will less likely be a victim.