There are quite a few cat choices, any of the synodontis cats, or large pictus-types like 4 line cats, but you'll want a group if you go the pictus route. For a big cat, in the 150, you could try a bullhead or maybe a lima shovenose, but you'll want to buy the lima well grown, they grow much slower than oscars.
Your senegal bichir should be OK with the oscar, but watch them as they grow, I have a senegalus I bought from Petsmart, he's almost 2 years old and barely 7" and thin. A decent size oscar could make a snack of him.
