Many surgeons believe that one anastomosis (mini) gastric bypass (OAGB/MGB) is associated with a high marginal ulcer (MU) rate and that this is associated with complications in a significant number of patients. The purpose of this survey was to find out the participant-reported incidence of MU after OAGB/MGB and its complications. We also aimed to understand practices in this cohort concerning prophylaxis, diagnosis, treatment and management of complications. Bariatric surgeons who perform OAGB/MGB procedures were invited to participate in a confidential, online survey using SurveyMonkey®. A total of 86 surgeons performing OAGB/MGB procedures participated in the survey. The total number of OAGB/MGB procedures reported was 27 672, revealing 622 MU, giving an MU rate of 2.24 %. Most participants (69/84, 82.4%) routinely use proton pump inhibitor (PPI) prophylaxis, but there was variation in drugs, dosages and duration. The majority (49/85, 57.6%) of participants ‘always’ use endoscopy for diagnosis, and 48.1% (39/81) ‘always’ perform an endoscopy to ensure healing. Most (49/55) perforated ulcers were treated with laparoscopic repair +/− omentoplasty +/− drainage. Most (55/59, 93.0%) of the bleeding ulcers were managed with PPI +/− blood transfusions +/− endoscopic intervention (23/59, 39.0%). Non-healing ulcers were treated by conversion to Roux-en-Y gastric bypass (RYGB) in 46.5% of patients (n = 20/43). The participants did not report any MU-related mortality but described a number of risk factors for it. This survey is the first detailed attempt to understand the incidence of MU following OAGB/MGB; its complications; and practices concerning prophylaxis, diagnosis, treatment and management of complications.