This study compares three preoperative strategies before one anastomosis gastric bypass(OAGB) for severe obesity. Participants will be assigned to receive either tirzepatide medication for 60 days, a ketogenic diet for 4 weeks, or standard care before surgery. The main goal is to determine which approach leads to the best weight loss one year after surgery. The study will also evaluate safety, surgical outcomes, and improvement of obesity-related health conditions such as diabetes, high blood pressure, and sleep apnea.