Bariatric surgery is the most effective long-term treatment to obesity, and it is necessary to assess changes in body composition and to be able to establish better follow-up of patients. Cross-sectional, observational study in patients undergoing One Anastomosis Gastric Bypass (OAGB) bariatric surgery. We analysed changes in weight and body composition during the first postoperative year. 405 patients (68.9% women. 31.1% men), mean age 44 years, mean weight 110.02 kg, Body Mass Index (BMI) 39.76 kg/m2, height 1.66 m. The variables analyzed were substantially decreased compared to the preoperative values one year after surgery in every case: weight (110.02 ± 22.03 kg vs. 69.36 ± 13.60 kg), BMI (39.76 ± 6.65 vs. 24.52 ± 3. 76), fat free mass (61.12 ± 12.43 kg vs. 53.61 ± 11.61 kg), fat mass (50.44 ± 14.36 kg vs. 15.74 ± 6.74 kg), bone mass (58.06 ± 11.85 kg vs. 50.92 ± 11.06 kg) and water (45.08 ± 9.99 kg vs. 37.39 ± 9.23 kg), P < 0.001. The results show noticeable improvements in weight reduction and changes in body composition, and will contribute to develop a thorough understanding of both of them, contributing also to perform a better patients' follow-up.