the absolute smallest tank to breed oscars in is a 125 with no exceptions. and thats still pretty small. oscars get quite violent towards each other when they're breeding, so first of all, you need 3 of them, which i wouldnt put 3 in less than a 125 anyway, the two that are sexing, and then one to distribute the aggression.
i dont think they'd breed in a 55. heh. 3 oscars in a 55 is retarded first off, and second, its in-humane.