<p>Obstructive sleep apnea (OSA) can cause intermittent cerebral hypoxia that may impair brain health. However, widely used clinical indicators such as the Apnea-Hypopnea Index (AHI) and peripheral oxygen saturation (SpO₂) may not accurately capture cerebral oxygen dynamics. In this study, we recorded frontal lobe cerebral oxygen saturation (rSO₂) using functional near-infrared spectroscopy (fNIRS) while concurrently acquiring overnight polysomnography (PSG) in 23 cognitively normal older adults (13 with OSA and 10 controls). Our results demonstrated that rSO₂ was negatively associated with apnea indices but showed no association with hypopnea indices. In contrast, mean SpO₂ was associated only with hypopnea indices, and minimum SpO₂ was associated with all indices. Applying increasingly stringent exclusion criteria to improve fNIRS signal quality strengthened the association between rSO₂ and apnea indices, but rSO₂ was not associated with hypopnea indices. These findings suggest that apneas cause cerebral desaturation, whereas hypopneas are mitigated by cerebral autoregulation, revealing a dissociation between cerebral oxygenation and SpO₂. Accordingly, SpO₂ alone may not fully reflect cerebral hypoxia. Thus, integrating high-quality fNIRS with PSG could provide clinically meaningful insights into both the physiological burden of OSA and its potential impact on cognitive function.</p>