This paper reviews the updated findings, including the current clinical guidelines and recent research trends, on the pathophysiology, diagnosis, and treatment of obstructive sleep apnea (OSA), a common sleep disorder associated with medical and mental disorders. The pathophysiology of OSA is believed to be related not only to the anatomical causes of the upper airway but also to the comparative anatomical mechanisms, such as the upper airway muscle function, arousal threshold, and loop gain. When OSA is suspected of being a clinical symptom or sign, a sleep study should be used for a diagnosis of OSA. Traditionally, continuous positive airway pressure has been the most effective first-line treatment for OSA. On the other hand, there could be a limitation of long-term compliance and new therapies that are suitable for the diverse mechanisms of OSA have been proposed or studied. In the future, the treatment of enhancing the effect through a more individualized consideration should be applied for OSA.