BACKGROUND: One anastomosis gastric bypass (OAGB) is increasingly used in the treatment of morbid obesity. However, the efficacy and safety outcomes of this procedure remain debated. We report the results of a randomised trial (YOMEGA) comparing the outcomes of OAGB versus standard Roux-en-Y gastric bypass (RYGB). METHODS: This prospective, multicentre, randomised non-inferiority trial, was held in nine obesity centres in France. Patients were eligible for inclusion if their body-mass index (BMI) was 40 kg/m(2) or higher, or 35 kg/m(2) or higher with the presence of at least one comorbidity (type 2 diabetes, high blood pressure, obstructive sleep apnoea, dyslipidaemia, or arthritis), and were aged 18-65 years. Key exclusion criteria were a history of oesophagitis, Barrett's oesophagus, severe gastro-oesophageal reflux disease resistant to proton-pump inhibitors, and previous bariatric surgery. Participants were randomly assigned (1:1) to OAGB or RYGB, stratified by centre with blocks of variable size; the study was open-label, with no masking required. RYGB consisted of a 150 cm alimentary limb and a 50 cm biliary limb and OAGB of a single gastrojejunal anastomosis with a 200 cm biliopancreatic limb. The primary endpoint was percentage excess BMI loss at 2 years. The primary endpoint was assessed in the per-protocol population and safety was assessed in all randomised participants. This study is registered with ClinicalTrials.gov, number NCT02139813, and is now completed. FINDINGS: From May 13, 2014, to March 2, 2016, of 261 patients screened for eligibility, 253 (97%) were randomly assigned to OAGB (n=129) or RYGB (n=124). Five patients did not undergo their assigned surgery, and after undergoing their surgery 14 were excluded from the per-protocol analysis (seven due to pregnancy, two deaths, one withdrawal, and four revisions from OAGB to RYGB) In the per-protocol population (n=117 OAGB, n=117 RYGB), mean age was 43.5 years (SD 10.8), mean BMI was 43.9 kg/m(2) (SD 5.6), 176 (75%) of 234 participants were female, and 58 (27%) of 211 with available data had type 2 diabetes. After 2 years, mean percentage excess BMI loss was -87.9% (SD 23.6) in the OAGB group and -85.8% (SD 23.1) in the RYGB group, confirming non-inferiority of OAGB (mean difference -3.3%, 95% CI -9.1 to 2.6). 66 serious adverse events associated with surgery were reported (24 in the RYGB group vs 42 in the OAGB group; p=0.042), of which nine (21.4%) in the OAGB group were nutritional complications versus none in the RYGB group (p=0.0034). INTERPRETATION: OAGB is not inferior to RYGB regarding weight loss and metabolic improvement at 2 years. Higher incidences of diarrhoea, steatorrhoea, and nutritional adverse events were observed with a 200 cm biliopancreatic limb OAGB, suggesting a malabsorptive effect. FUNDING: French Ministry of Health.