Olanzapine scares the crap out of me — with my host of physical disorders I can’t take the stuff. From what I’ve read it can cause weight gain or weight loss.
I don’t know if you’ll lose weight if you cease taking your meds. I believe that if I were you, I would discuss your concerns with your doctor before you stop taking your meds. S/he has you on an antipsychotic for a reason.
Good luck.