IME they were good tankmates but I also have them in a 180gallon. My female convict just get aggressive when she lays eggs and only attack my Oscar when it gets too close to her eggs.
My Oscar is about 5 times bigger. My Oscar is not scared of my Convict its just that my convict is much faster, so she let's her be and move along. When my convict doesn't have babies they just ignore each other.
If ur afraid convict might get eaten, get a male they get bigger. And have a cave for the convict cichlid that the Oscar won't fit into.