Aim: Obstructive sleep apnea (OSA) is characterized by repetitive upper airway collapse during sleep, resulting in frequent cortical arousals. However, currently used frequency-based arousal metrics do not sufficiently capture the heterogeneity and clinical significance of arousal responses. The odds ratio product (ORP) is a novel electroencephalographic marker that provides a continuous assessment of sleep depth and has the potential to serve as an objective measure of arousal intensity.<br /> Purpose: This study aimed to quantify the intensity of arousals in untreated OSA patients using the ORP, and to explore the relationships between arousal intensity, respiratory event features, and subjective sleepiness.<br /> Patients and Methods: We retrospectively analysed data from 1057 adults with untreated OSA enrolled in the APPLES cohort. EEG spectral power was mapped to ORP values, and arousal intensity for each event was objectively calculated based on deviations in ORP from baseline. A total of 258,121 arousal events were included. Mixed-effects modelling was used to assess the impact of event type, duration, latency, sleep stage, position, and inter-individual variability on arousal intensity. Stepwise multiple regression explored associations between individual arousal intensity and subjective sleepiness.<br /> Results: Arousal intensity increased significantly with the duration of preceding respiratory events, and was markedly higher than that of spontaneous arousals. The association between respiratory events and arousal intensity was stronger for apneas than for hypopneas, while deep sleep stage and lateral posture significantly reduced arousal response. Inter-individual variability was pronounced. Higher baseline arousal intensity was independently associated with increased subjective daytime sleepiness, after adjusting for known confounders.<br /> Conclusion: ORP-derived arousal intensity provides a quantitative biomarker of cortical arousal. Arousal intensity is shaped by respiratory event characteristics, sleep architecture, and intrinsic individual traits. Although slight, arousal intensity is independently associated with subjective daytime sleepiness.