Depending on tank size and the size of the oscar, but possiable easy to find tankmate could be Jack dempsy, Green terror ( gold saum), Texas, convict, salvini, black belt.
I think oscars go well with most fish that won't fit in its mouth as said previously, but they are usually territorial if they have been the only fish in the tank since being thrown in there.. if they grow up with it, it usually ends up ok.. plecos, fire eel, or texas seem to work well. I think JD's are too aggressive for oscars personally.. Keep size of the tank in mind too..
depending on the size of the tank, but generally most not so aggressive non-dwarf South American species can be ok with them like chocolates, severums, large geos, large acaras, also smaller Central Americans, as long as you exclude the large aggressive species
Depends on tank size but if it's big enough you could put a giant gourami with an oscar. Not too many people seem to dig that fish but I always liked it.
The problem I had with my oscars is they were really territorial and stopped everyone going past mid-tank. My jd's, gts all hid most of the time due to them; having said that my biggest oscar attempted to murder a same sized sailfin pleco..
I believe JD's arn't as aggressive as oscars, though the difference is a JD will do more damage since they actually have teeth unlike the 400grit sandpaper-mouth of an oscar.
In a nutshell my experience with oscars has left me believing they have the aggression of a lot of mean fish but can't back themselves up when it comes to liplocking - so be careful if you get ie a blackbelt, texas because if they get in a bad mood they would murder the oscar very quickly.
After all that I still have no idea what a suitable tankmate for a large oscar is... silly creatures they are!