My Oscar had one of my Apple Snails in his mouth for 3 days, he would occaisonally drop it, then snatch it up again. I wouldnt trust them with anything under 6''.
I don't see that eating a crab would hurt the oscar. Oscars like all cichlids have pharyngeal teeth. These teeth are for the purpose of processing crustaceans and other critters with exoskeletons.
well i fed the crab to my oscar, but before it even hi the bottom of the tank my convicts tore into it! so i guess a oscar will eat a crab if no one else gets to it first