Background <p>Prevention of hypoxemia in obese patients undergoing sedated gastroscopy is challenging, mainly due to upper airway obstruction. This study used a transorally inserted supraglottic endotracheal tube as an oropharyngeal airway to improve ventilation during sedated bidirectional endoscopy (esophagogastroduodenoscopy followed by colonoscopy) in patients with obesity.</p> Methods <p>Patients with mild-to-moderate obesity (body mass index: 30–39.9&#xa0;kg/m²) underwent bidirectional endoscopy during deep sedation and received supplemental oxygen via a modified oropharyngeal airway using an endotracheal tube (ETT-OA) (<i>n</i> = 97) or a nasopharyngeal airway (<i>n</i> = 96). An endotracheal tube was inserted through the side port of the endoscopic bite block into the supraglottic area under gastroscopic guidance. The primary outcomes were the incidence of hypoxemia and severe hypoxemia.</p> Results <p>The minimum SpO₂ was comparable between the two groups. The incidence of hypoxemia was significantly lower in the oropharyngeal airway group than in the nasopharyngeal airway group (4.1% vs. 14.6%, <i>P</i> = 0.012). Severe hypoxemia occurred in six patients in the nasopharyngeal airway group but not in the other group (<i>P</i> = 0.037). The distal end of the nasopharyngeal airway was not inserted into the supraglottic area in eight patients because the tube was short and could not be replaced with a larger-sized tube due to rhinostenosis. Epistaxis occurred in 12 (12.5%) patients in the nasopharyngeal airway group. Anesthesiologists and endoscopists were more satisfied with the oropharyngeal airway.</p> Conclusions <p>An ETT-OA reduced the incidence of hypoxemia, especially severe hypoxemia, in patients with mild-to-moderate obesity during sedated bidirectional endoscopy compared with a nasopharyngeal airway.</p> Trial registration <p>Chinese Clinical Trial Registry (ChiCTR2200064417), first registered on 07/10/2022.</p>