Obstructive sleep apnea (OSA) is a very common but potentially serious sleep disorder. OSA is characterized by repetitive partial or complete cessation of airflow due to the closure and re-opening of the upper airway during sleep. Therefore, OSA patients experience rapid fluctuations in oxygen concentration during sleep, increasing their risk for cardiovascular diseases (CVD), metabolic syndrome, and neurocognitive problems. Various screening tools have been developed for the early detection of OSA, including anatomical indices such as the Mallampati and Friedman indices, as well as questionnaires including the Epworth Sleepiness Scale and STOP-Bang. However, due to the limited diagnostic accuracy of these methods, novel methods for screening OSA are still highly needed. A recent review article summarizing the available evidence on OSA biomarkers suggested that an optimal screening test should be clinically sensitive, specific, simple, timely, inexpensive, and correlated to disease severity. Furthermore, biomarkers should be able to reflect the functional changes that accompany OSA, as well as making pathophysiological sense. Thus, many researchers have sought to find biomarkers that satisfy the aforementioned criteria using various biological fluids, including blood, urine, and saliva. Here, we would like to share the research results and the necessity of developing new diagnostic markers that reflect the pathophysiology of sleep apnea.