I got a jag about 4in and an oscar close to 6in. Of course the oscar is alot bulkier do to being the heavy bodied fish they are (Jags more streamline). They have quarrelled a bit, mouth to mouth fighting, and a little chasing. But they are an even match and the fighting is infrequent. I'd say depends on the temperment of your fish. Oscars are usually not very aggressive for cichlids, and so having a larger oscar (not too big) than an aggressive species, such as a jag, is a plus. That being said I have never owned this combo of fish in the past, so im not too sure what to expect. Just putting it out there
