Background <p>Of patients suspected as having obstructive sleep apnea (OSA) while undergoing routine gastrointestinal endoscopy under conscious sedation (GE-CS), 43% have severe disease but have never consulted specialist clinics because of the absence of significant symptoms/signs. If patients with possible severe OSA can be predicted during GE-CS, they could be advised to consult sleep specialist clinics.</p> Materials and Methods <p>We retrospectively analyzed data of 112 patients who were suspected as having OSA during GE-CS and underwent sleep apnea testing at our hospital, to evaluate whether patient characteristics, endoscopic detection of upper airway obstruction (UAO), and vital signs during routine GE-CS can predict the possibility of severe underlying OSA.</p> Results <p>The mean age of the patients was 55&#xa0;years, 74% were men, and 64% had obesity. Severe OSA was diagnosed in 13% of patients with mild UAO (&lt; 50% luminal stenosis), 18% of patients with moderate UAO (50%-75% stenosis), and 45% of patients with severe UAO (&gt; 75% stenosis). The UAO severity was positively correlated with the apnea hypopnea index score (r = 0.305, <i>p</i> = 0.001) and negatively correlated with the minimum SpO₂ during GE-CS (r = 0.285, <i>p</i> = 0.023). Logistic regression analysis identified male gender (HR 4.19, <i>p</i> = 0.0267), severe UAO (HR 3.69, <i>p</i> = 0.009), and minimum SpO₂ &lt; 90% during GE-CS (HR 3.82, <i>p</i> = 0.008) as an independent predictors of severe OSA.</p> Conclusion <p>Not only the presence of UAO, but also minimum SpO₂ &lt; 90% during GE-CS was identified as an independent predictor of severe OSA.</p>