Middle ear negative pressure and effusions have been described after oxygen diving. The prevalence, dynamics and pathophysiology of this phenomenon are not clear, and were hence investigated in the present study. Thirty-four oxygen divers with normal otoscopic and tympanometric evaluation participated in the study. The subjects' symptoms were documented, and pneumatic otoscopy and tympanometry were repeated immediately after the completion of a 3 h, 15 feet oxygen dive, and 7 h later on awakening from the night's sleep. Most divers had positive otoscopic findings the morning after the dive, all of which cleared within 4 h of rising. A significant decrease was observed in average middle ear compliance (p = 0.0463, one way ANOVA), and an increase was found in the number of ears with tympanic compliance less than 0.3 ml (p = 0.0001, Kruskal-Wallis non-parametric ANOVA). In addition, 14.7% of the ears had type C, and 27.9% type B tympanograms the morning after the dive (p = 0.0001 chi2). The generalized nature of oxygen-induced middle ear under-aeration, combined with the dynamics of the symptoms and signs observed, make middle ear barotrauma, tympanic cavity oxygen absorption, and middle ear epithelial oxygen toxicity all unlikely explanations. The observed phenomenon and its dynamics might stem from a reversible derangement in a middle ear chemoreceptor reflex arch, which has recently been suggested as regulating middle ear aeration.