Aim Epidemiological studies have shown that severe obstructive sleep apnea (OSA) is associated with higher mortality when compared with mild to moderate OSA. Because aging is a well‐known risk factor for OSA, we aimed to elucidate the underlying factors associated with the severity of OSA in elderly patients. Methods Patients who underwent polysomnography were divided into the non‐elderly group (aged <65 years; n = 44) and the elderly group (aged ≥65 years; n = 46). The severity of OSA was determined by the apnea hypopnea index (AHI), and each group was subdivided into two groups: mild to moderate OSA (5 < AHI < 30) and severe OSA (AHI ≥30) . In the elderly group, geriatric assessments to evaluate physical and neuropsychiatric function were carried out. Results All patients had OSA as diagnosed by an AHI >5. Whereas body mass index was positively correlated with AHI in both groups, age was correlated with AHI only in the elderly group. Body mass index and age were higher in severe OSA than mild to moderate OSA in the elderly group. Unexpectedly, no significant difference was observed in physical strength, cognitive function, apathy scale, depression scale or activities of daily living between mild to moderate OSA and severe OSA in the elderly group. Binary logistic regression analysis showed that male sex, body mass index and aging were independent risk factors of severe OSA in the elderly group. Conclusions Our findings suggest that aging increases the severity of OSA in elderly patients, even if they are physically active and neuropsychiatrically unimpaired. Geriatr Gerontol Int 2017; 17: 614–621.