Convicts are the perfect tank mate for an oscar. Severums, firemouths, chocolates are all good to. Try to keep the temperment on the same level as an oscar (semi aggressive). A jd & green terror can work to, but their growth rates curbs at 6" while the oscar grows 1"+ a month till 10". Anything that will fit in eats mouth will end up there.
You "might" be able to get away with a female jag...MIGHT. I'd try to stay away from parachromis/amphilophus species to be honest & larger CA's in general. A male jag or dovii will be a death sentence for an oscar.
I've kept a severum, convict, a black ghost knife, jd, and a feather fin syno with different oscars no probs except the occassional hospital tank trip for the smaller mates.