Oscars are a neutral to soft water fish from South America, from east of the Andes.
So the best tank mates would be other South American fish from that geographic area.
Fish to avoid would be more aggressive cichlids from the hard waters of Central America, and west of the Andes, even some smaller yet aggressive ones.
IMO opinion, this info prevents, JDs, red or green terrors, Texas cichlids, and any of the convict types as good tank mates, it would also eliminate rift lake African cichlids.
Oscars appreciate sluggish, low flow waters,
This might also eliminate any rheophillic species, and of course, any fish that fit into its large mouth.
Appropriate tank mates would be tetras, some less aggressive cichlids and catfish from S America "too large" to be swallowed, but also that don't get so big they compromise water quality in the oscars tank.
If water quality is compromised by high nitrate, and the metabolic substances, oscars tend to get scarred up from disease like HLLE/ hoie in the head, and other chronic maladies.