I wouldn't go with clams, Oscars are just to inquisitive and would probably try to eat them.
As for the crayfish, I wouldn't try to keep one as a pet. I get crayfish for my Oscars about once a month for treats, they are usually small crayfish and I remove the claws, but they only last seconds in the tank. Given the chance though, the crayfish will nip at the fins of your Oscar, specially at night while your Oscar is resting, the crayfish will is nocturnal and will terrorize the Oscar during the night.