Background: One anastomosis gastric bypass/mini-gastric bypass (OAGB/MGB) is considered to be the best alternative to Roux-en-Y gastric bypass due to the shorter operative time and fewer possible complications. Objectives: The purpose of this study is to determine biliary reflux in patients who undergo OAGB/MGB with a hand-sewn gastroenteroanastomosis, and OAGB/MGB with a linear stapled gastroenteroanastomosis. Materials and Methods: This prospective-retrospective study includes the results of treatment of 122 obese patients who underwent laparoscopic OAGB/MGB. Patients were divided into two clinical groups depending on the method of performing gastroenteroanastomosis. In the postoperative period, the possibility of developing biliary reflux was assessed using fibrogastroscopy with biopsy of the gastric mucosa. Histopathology of biopsy materials was evaluated. Microscopy of biopsy specimens was performed by a pathologist who evaluated each sample of gastric tissue in accordance with the system for determining the biliary reflux index (BRI). Results: With fibrogastroscopy, 9 (15.5%) cases in the study group and 16 (26.6%) cases in the control group, biliary reflux was diagnosed. In the study group of patients, BRI >14 was determined in 3 patients (5.153%). In the control group of patients, BRI >14 was found in 7 cases (10.94%). The difference is significant (p < 0.05). Conclusion: Determination of the BRI index in patients undergoing an OAGB/MGB can be used as a screening method to diagnose biliary reflux for its prevention and the development of tactics for the further management of patients with a high risk of complications associated with the toxic effects of aggressive bile. Hand-sewn gastroenteroanastomosis carries less risk of bile reflux gastritis and anastomositis than stapled gastroenteroanastomosis. Clinical Trial Registration number NCT04845438