there are some things that you can do to decrease agression, the frist thing you have done is to seperate them, I am not sure which fish you took out but often IME many people take the fish thats getting picked on out and with cichlids that are in the general sense peaceful like sevs and oscars, people need to take the bully out, by doing this espcially since the oscar is the new guy to the tank you give him a chance to set up a territory, the second thing is to keep the temp down around 76 degrees F by decreasing the temp you lower the metabolism of the fish and so you lower their agression. but I would take the sev out and put him back in his old tank for a few days totally rearrange all of the tank decos so it looks nothing like the orginal and then lowering the temp you will give the O a chance and then in a couple of days reintroduce the sev and everything should in theory go much better, but keep an eye on them!!!! also the addition of some dithers would never hurt.