im willing to bet a green terror or a jd got him if he looks like some one took sand paper to him hes dying, and for those that think they are tuff they are mistaken oscars are a big beautiful smart fish but cannot handle the aggression of gts and JDs, also i heard of other oscars teaming up as a pair and knocking the third out, AND YES they have personality and definately may be sulking ....but like some one said a lil salt and half the recomended melafix may work i have seen similar fish with just salt come back and i think the melafix is too much chemical for a seriously near death fish and it kills them ... Once he is upsidedown but still breathing its over then its time for the freezer .. : ( very sad i hope he recovers for u ...sorry to hear it, once he recovers coupel weeks if u put him back in you will have to keep a close eye, may hae jsut been a turf disput and he lost to a JD or a GT.. again good luck