if the oscar is now seperated, just nice clean water will heal him right up. also don be suprised if the black belt starts targeting your other fish too. in my experience black belts become regular a-holes the bigger they get.
oscars are big softys and can rarely stay with any aggressive fish, the oscar or blackbelt will prob have to go mate... the rest of the stock are a little more aggressive and could prob hang with the bb until you upgrade.
The only luck I've had consistently is a time out long enough for the smaller fish to catch up in size. Typically months 1-2. Had to qt my jag for 3 to get everyone up to size enough.
Now everyone's on a more level playing field.
It would be hard with a Midas either way as they grow pretty fast. Short timeouts have never worked for me