"O" sounds like a jerk! Are you alright?
Sit down with your boyfriend right away and explicitly tell him that he *will not* deal with "O" in a physical manner. His brothers will follow his lead since he is, for want of a better term, the injured party.
He might secretly think that, despite what you've told him, you want him to beat the poop out of "O" so make sure he knows that just the opposite is true.