Obstructive sleep apnea (OSA) and 25-hydroxyvitamin-D₃ (25-OH-D) deficiency are two separate disorders associating with obesity, inflammation, and impaired glucose metabolism. We aimed to investigate the vitamin D status of OSA patients regarding to potential links between lower vitamin D levels and abnormal glucose metabolism, which is one of the main adverse outcomes of OSA. Study group is composed of 190 non-diabetic subjects who were suspected of having OSA. Subjects undergone polysomnography and were grouped due to apnea-hypopnea indices (AHI) as controls (AHI < 5, n = 47), mild OSA (5 ≤ AHI < 15, n = 46), moderate OSA (15 ≤ AHI < 30, n = 47), and severe OSA (AHI ≥ 30, n = 50). Serum 25-OH-D, HbA₁c, insulin levels were measured and 75-g oral glucose tolerance test was performed. Serum 25-OH-D level (ng/ml) of OSA patients were lower than control subjects (17.4 ± 6.9 vs. 19.9 ± 7.8), and decrement was parallel to severity of OSA; as 18.2 ± 6.4 (5 ≤ AHI < 15), 17.5 ± 7.4 (15 ≤ AHI < 30), and 16.3 ± 6.9 (AHI > 30), respectively (P = 0.097, r = -0.13). However, severe female OSA patients had significantly lower 25-OH-D levels (11.55 ng/ml), while control males had the highest mean value (21.7 ng/ml) (P < 0.001). Frequency of insulin resistance (IR) was 48%, prediabetes 41%, diabetes 16% in OSA patients. Mean 25-OH-D level of insulin resistant subjects (HOMA-IR ≥ 2.7, n = 77, AHI = 35.5) was lower than non-insulin resistant subjects (HOMA-IR < 2.7, n = 113, AHI = 19.8) as 16.18 ± 7.81 versus 19.2 ± 6.6, respectively (P = 0.004). 25-OH-D level of 91 non-diabetic subjects (n = 91, AHI = 19.7) was 19.5 ± 7.4, prediabetics (n = 75, AHI = 28.7) was 17.45 ± 6.9, and diabetics (n = 24, AHI = 46.3) was 13.8 ± 5.3 (P = 0.02). We showed that subjects with more severe OSA indices (AHI ≥ 15) tended to present lower vitamin D levels correlated to increased prevalence of IR, prediabetes, and diabetes. Vitamin D deficiency may play a role and/or worsen OSA's adverse outcomes on glucose metabolism. OSA patients may be considered for supplementation treatment which was shown to ameliorate abnormal glucose metabolism and inflammation.