I would not trust the oscar to not try to eat the eel. The eel is slender enough to fit in the oscars mouth. Whether the oscar succeeded in totally swallowing the eel or not the eel would still die as a result of the attempt. But maybe fire eels have some defense mechanism I am not aware of?
Beasts
I would imagine that t'would be alright to put 'em in together. fire eels are good at hiding and even if the oscar would get to it, it probably wouldnt be able to swallow it whole.
Now where was this that people viewed a video of a large grouper trying to make a dinner out of a young tess moray and wasn`t able to swallow it all and had to spit it out and the eel died. So just how tough is your fire eel? Also all eels hide and if your any aggressive predator in the tank, it would have to remain in hiding or you can play "taps" in which no one wants to :headshake2:
A 10" Oscar WILL eat an 8" eel. If it can't swallow it whole, it will swim around with the eel hanging out of its mouth until part is digested, then swallow the rest. I've seen the same thing with juvie Oscars and snakeheads, though it was many years ago, long before they were banned.