What kind of clean-up crew can I have with a tiger oscar?

I have him in a 20 gallon alone (he is small still, and I plan on upgrading to a bigger tank.) I had snails in there but they all disapeared "mysteriously." I put a ghost shrimp in from an existing tank, but that also disapeared. This is my first time with oscars, or, for that matter, any kind of chichlid, and would appreciate any help. Thank you in advance for any answers.