Objective: To evaluate retinal nerve fiber layer thickness (RNLF) with optical coherence tomography (OCT) and optic nerve function with visual evoked potential (VEP) in patients with obstructive sleep apnea syndrome (OSAS). Methods:Fifty-one eyes of 30 newly diagnosed OSAS patients who received no treatment were included. The RNFL analysis with OCT and pattern VEP test (120',30',15',7' pattern size) was performed to all patients. Results:Of 30 patients, 20 were females and 10 were males. None of the patients were obese with a body mass index of (BMI) < 30 kg/m 2 . The mean Apnea Index of the patients was 42.5 ± 24.3. All patients had severe OSAS with an Apne Index of ≥ 30. Visual acuity was 20/20, intraocular pressure and optic disc were within natural limits in the all of the eyes included in the study. A statistically significant decrease (p < 0.05) was detected in the all parameters of p100 wave amplitude of VEP tests. Compared to the standard data of the healthy individuals at the same age, there was no significant difference in the p100 latency. Although there was thinning of the RNFL thickness, there was no statistically significant difference (p > 0.05). Conclusion:In patients with OSAS, ischemia during deep sleep causes optic nerve dysfunction which can be detected by electrophysiological test, even in asymptomatic patients. In this study, the importance of early diagnosis and treatment is emphasized, and particularly early treatment may halt the progression of the optic nerve dysfunction. For a better understanding of these diseases and ocular manifestations, further long-term studies are needed.