I don't drink any water except a small cup because i have oab how bad is this for my body?

Find out the cause : OAB is not uncommon but may be a sign of other illnesses such as anxiety, bladder infection, neurologic or urologic problem. Fluid restriction only helps to relieve the symptom but not a cure. Too much fluid restriction may be harmful to your body. I suggest you talk to your doctor and request an appropriate referral to find out the actual cause of your OAB for cure. .