I'm sorry that you're having to deal with a negative person like that. It's unfortunate that you had to drop a class because of this jerk.

What exactly has O been saying to your mother? If death threats are being made, I think you should contact authorities. It would be horrible if he ended up hurting someone or even himself.

As for yourself, I think you should try to distract yourself with things you enjoy to make yourself feel better. Also, talking to friends about this (or even people online, which you're doing right now) is a really good idea and it can help you a lot.

Seriously though, you should report these threats if they're really violent. There's a big chance that O's threats are empty ones, but it's better to be safe than sorry.