festaedan & Crazy mike are right. thing is, whether the oscar is scraping on decor (seems less likely), or getting straight up bitten, doesn't really matter in your dilemma. oscars are no match for midas and jag aggression.
not uncommon for fish to wait until after dark or whenever owner isn't there, to whack tankmates.
not uncommon for fish to wait until after dark or whenever owner isn't there, to whack tankmates.