AIM: To detect retinal microvascular variations in obstructive sleep apnea syndrome patients. METHODS: This prospective, observational case-control study included healthy controls and patients with mild, moderate, and severe obstructive sleep apnea syndrome. Vascular parameters, foveal avascular area, and flow areas in macula-centered, 6.00×6.00 mm2 scan size optical coherence tomography angiography images were compared. RESULTS: The control group had the highest whole image, parafoveal, and perifoveal vessel density among the groups in both superficial and the deep capillary plexus (all P<0.05). Rapid eye movement sleep apnoea-hypopnoea index was reversely correlated with whole (Rho=-0.195, P=0.034), parafoveal (Rho=-0.242, P=0.008), perifoveal (Rho=-0.187, P=0.045) vessel density in the superficial capillary plexus, and whole (Rho=-0.186, P=0.046), parafoveal (Rho=-0.260, P=0.004), perifoveal (Rho=-0.189, P=0.043) vessel density in the deep capillary plexus, though the mean and non-rapid eye movement sleep apnoea-hypopnoea index related with only parafoveal vessel density in the superficial capillary plexus (Rho=-0.213, P=0.020; Rho=-0.191, P=0.038) and the deep capillary plexus (Rho=-0.254, P=0.005; Rho=-0.194, P=0.035). CONCLUSION: This study shows decreased vessel density and its reverse correlation with the apnoea-hypopnoea index in patients with obstructive sleep apnea syndrome.