Obstructive sleep apnea (OSA) disrupts sleep. This study examined factors related to OSA severity. A cross-sectional, prospective, hospital-based study was conducted with 205 patients who underwent polysomnography (PSG). Demographic, anthropometric, clinical, PSG, and sleep quality assessment data were analyzed. Participants (N = 205) were classified into four groups based on apnea–hypopnea index (AHI); no OSA (AHI < 5/h; N = 14), mild (mOSA, 5 < AHI < 15/h; N = 50), moderate (modOSA, 15 < AHI < 30/h; N = 41), severe (sOSA, 30 < AHI < 60/h; N = 50), and very severe (vsOSA, AHI ≥ 60; N = 50). Men had more severe OSA than women (p < 0.001). Anthropometric characteristics differed with OSA severity (p < 0.001). OSA patients had decreased sleep quality and increased excessive daytime sleepiness (EDS). Body mass index (BMI), neck/waist circumference, and blood pressure (BP) differed between groups (p < 0.001). Patients with vsOSA had the highest Mallampati grades (p < 0.001). Multiple linear regression indicated that OSA severity was related to gender and sleep quality. PSG parameters (oxygen saturation, systolic BP, and arousal/respiratory arousal) were strongly related to OSA severity. In conclusion, about half of study-referred patients had severe/very severe OSA; these groups were predominantly obese men with high BP. OSA severity associated with high BP, BMI, waist circumference, and neck circumference.