Well oscars are normally quite peaceful but during feeding can get quite aggressive. Im not familiar with flag cichlids at all but i understand they are a more peaceful south american cichlid (correct me if im wrong) and severums are generally pretty mild mannered as well. You may try throwing a few more dither fish (aka your silver dollars) to try and distract any aggression towards the cichlids to the dithers. But u may wanna try donating ur current stock and restocking in accordance with what you truly want and love. If your really into ur oscar id remove the others and just let him grow, maybe get 2 more to have a few focal fish rather than just the one. Most the fish stores i go to have no guarentee on any of their cichlids for this reason in particular. Every fish is different and dont be surprised when one acts completely opposite of how everyone describes the species.