 The impact of different severities of obstructive sleep apnea (OSA) on cognition has not well been documented. Neuroglobin, a recently discovered vertebrate protein, shows a neuroprotective function to modulate hypoxic–ischemic brain injury. This study investigated the cognitive function of patients with varying OSA severities and intended to explore the relationship between levels of neuroglobin in serum and cognitive deficits of OSA.