the oscars get about 8-10" bigger than the jewels, but on the other hand the jewels are more aggressive. i never had much luck with an oscar in a mixed chiclid tank. the oscars are not as agressive, but grow ALOT faster. i've tried to start the oscar larger than the other fish but he out grew them and began taking over the whole tank. i've also tried to get them the same size and the oscar had no chance. the fish that went well with jewels was firemouths, jack dempsy's, convict chiclids and green terror. all about the same size and equally agressive.