Obstructive sleep apnea (OSA) is a sleep-related breathing disorder characterized by obstructions of the upper airway during sleep, resulting in repetitive breathing pauses accompanied by oxygen desaturation and arousal from sleep. OSA can be successfully treated with continuous positive airway pressure (CPAP), weight loss, positional therapy, oral appliances, hypoglossal nerve stimulation, and surgical procedures. It has been observed that untreated OSA is related to chronic disorders including hypertension, arrhythmias, congestive heart failure, coronary heart disease, diabetes mellitus, hyperlipidemia, stroke, depression, and cognitive decline. Event-related potentials (ERPs) is the procedure that has been widely used for evaluating cognitive brain functions. Using auditory event-related potentials (P300), this study aimed to examine the effect of CPAP therapy on cognitive functions in patients with moderate and severe OSA. The results of the study showed improvement in P300 latency and amplitude after 3 months of CPAP therapy, indicating a positive effect of CPAP therapy in the prevention of cognitive decline in patients with OSA.