I was asked to be the Maid of Honor by my one friend. Only catch is, she's marrying my ex whom Im not on good terms with. I want to be her MOH but I don't want to make him upset on his day also. How to handle the situation.

Why would he be upset?
I'm sure he knows you have been asked to be MOH so if he has no problem with it, you shouldn't.

OK I guess I'm just not ready to see him again its been awhile I hang with tiff when he's not there

Then you need to bow out of the wedding.
Does this girl know he is your ex?
Does she know you still have feelings for him.

I don't still have feelings for him. We hate each other and yes she knows

Well if you hate each other why are you worried about upsetting his day? Sounds like something else going on.
If she is your friend do the wedding. You'll see him a couple of times and that will be it.

OK ty

I'd tell the bride you'd rather just attend as a guest because you would feel awkward being maid of honor at a former boyfriend's wedding. You really shouldn't be maid of honor unless you wholeheartedly support the marriage.

I assume the bride knows this guy is your ex. She presumably has talked with him about it, and if it was a big issue to either of them, she likely wouldn't have asked you. Unless being around him is unbearable for you, you should tell her your concerns but remain open to the idea of being MOH. Ultimately, the day is about your friend and she wouldn't have asked you unless she felt you were the best person for the job. I'd try to endure the discomfort for her sake and agree to be MOH.

I will have to assume the bride knows he was your ex, and I will assume you have stayed friends with the bride even though she has been with your ex.

But hate is an emotion and it means you are not over him yet, not caring is over him.