Corys will never be big enough not to fit in a Oscars mouth after it hits around 4"
Not just that, if a Oscar trys to eat a Cory at that size it will probably die trying to eat it because they have that trigger like locking spine that will become wedged in a Oscars throat.
My local Maiden head had a adult fully grown striped Dora (they didn't realise it was in the tank hiding in a large piece of bog wood)
They put in the tank a 3ft Gar and 3ft Tiger Shovel Nose, the Shovel nose tried to eat the Dora and died in the progress, having it wedged in its throat.
To add to that i was recently hoovering out sand of my tank (so i could put black gravel in its place) with a hose pipe and i sucked up one of my corys by accident, the cory triggered his spine immediately and got stuck at the start of the tube, i stopped the syphon and shook him out, hes got scars but hes still alive now, saved by his spine, otherwise he would of gone out of the window into the snow.