Obstructive sleep apnea (OSA) disrupts sleep, leading to cognitive impairment and sleepiness, but risk stratification remains unclear. We developed an EEG-based arousal marker to quantify sleep fragmentation and assessed its associations with neurocognitive function and daytime sleepiness.Arousal intensity was derived from EEG signals in 1070 APPLES (Apnea Positive Pressure Long-term Efficacy Study) study participants. We analyzed its associations with PSG parameters, cognitive function (Wechsler Abbreviated Scale of Intelligence, WASI), and sleepiness (Multiple Sleep Latency Test, MSLT; Epworth Sleepiness Scale, ESS). Stepwise regression identified predictors of cognitive function. Sensitivity analysis examined arousal intensity's link to 6-month CPAP outcomes (N = 419).Among OSA individuals, the AHI ranged from 10 to 130.7/h, the average arousal index was 26.75/h and a total of 235,835 arousal events were extracted. The arousal intensity was significantly greater for hypoxia-associated and desaturation-related events compared to hypopnea-associated or spontaneous arousals (p<0.001). It correlated negatively with age, PSG measures, and cognitive function. Adjusted models showed Delta-band arousal intensity negatively predicted WASI-Full scores (β = -3.24, p = 0.019). Higher Gamma-band intensity was associated with both greater subjective sleepiness (ESS: β = 0.03, p = 0.028) and increased CPAP-induced MSLT improvement (β = 0.12, p = 0.020).Arousal intensity is a novel EEG marker for cognitive impairment and sleepiness in OSA, with potential clinical utility in predicting CPAP response. This EEG-based marker could be integrated into standard sleep analysis platforms to identify OSA patients at risk of cognitive impairment, enabling early intervention.Copyright © 2025. Published by Elsevier B.V.