once you thaw out the frozen worm, theres no need to mash it up, unless the fish is extremely small. even then, just cut it into a few bite-sized pieces.
once they have the taste for earthworms, they will sit there for an hour with a giant worm hanging out of their mouth trying to get it down (not a good idea, but still funny to watch)
try getting frozen tilapia fillet from a wal-mart. its dirt-cheap and when cut up, it will be a wonderful addition to your oscars diet.