How big are your current fish? Are they fully grown or still small? Yeah, I agree with above, if your fish are already grown and big, make sure to get another fish who is too big to fit in their mouth.
Senegal or delhezi would also work. Just make sure it is large enough not to get eaten. Giant danios work for dithers. Clown loaches are very interesting, they would work. Although the oscar might eat them eventually, so maybe thats not a good idea.