This study proposes a frequency-domain index of oxyhemoglobin saturation. We evaluate its reliability and compare with those of previous frequency-domain indices for screening the obstructive sleep apnea syndrome (OSAS). Patients diagnosed with OSAS by standard polysomnography were recruited Hospital Centre. Overall, 257 patients are used for training and 279 patients are used for validation. The presence of OSAS is defined as apnea-hypopnea index (AHI) > 5/h. Moderate and severe OSAS are defined as AHI > 15/h and AHI > 30/h, respectively. The proposed oxyhemoglobin index outperforms previously proposed frequency-domain indices. For predicting the severity of OSAS, the proposed index has a sensitivity/specificity of 74.3% / 96.1% for AHI > 15/h and 87.8% / 92.4% for AHI > 30/h. The results reveal that the proposed oxyhemoglobin index has a significantly smaller standard error of AHI than those of previous indices.