Mini-gastric bypass/One-anastomosis gastric bypass (MGB-OAGB) is an effective bariatric technique for treating overweight and obesity, controlling and improving excess-weight-related comorbidities. Our study evaluated OAGB characteristics and resulting weight evolution, plus surgical success criteria based on various excess weight loss indicators. A prospective observational study of 100 patients undergoing OAGB performed by the same surgical team (two-year follow-up). Surgical characteristics were: surgery duration, associated complications, bowel loop length, hospital stay, and weight loss at 6 postoperative points. 100 patients were treated (71 women, 29 men); mean initial age was 42.61 years and mean BMI, 42.61 ± 6.66 kg/m2. Mean surgery duration was 97.84 ± 12.54 minutes; biliopancreatic loop length was 274.95 ± 23.69 cm. Average hospital stay was 24 hours in 98% of patients; no surgical complications arose. Weight decreased significantly during follow-up (P < 0.001). Greatest weight loss was observed at 12 months postsurgery (68.56 ± 13.10 kg). Relative weight loss showed significant positive correlation, with greatest weight loss at 12 months and %excess BMI loss > 50% achieved from the 3-month follow-up in 92.46% of patients. OAGB seems to be effective in treating obesity, with short hospital stays. Relative weight loss correlates optimally with absolute outcomes, but both measures should be used to evaluate surgical results.