Background/Aim. It is necessary to find eligible oxidative stress markers for predicting the severity of obstructive sleep apnea hypopnea syndrome (OSAHS), a sleep disorder-related respiratory disease. We aimed to explore the correlation between oxidative stress and cognitive impairment in OSAHS patients. Methods. A total of 220 eligible patients were divided into snoring, mild to moderate OSAHS and severe OSAHS groups according to polysomnography (PSG). Apnea-hypopnea index (AHI), oxygen desaturation index (ODI) and baseline data were monitored. Oxidative stress indices were measured by colorimetry in early morning. They were divided into normal cognitive and cognitive impairment groups based on Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA). Independent risk factors for cognitive impairment were analyzed by multivariate logistic regression. Correlation between oxidative stress and cognitive impairment was analyzed by Pearson?s method. Receiver operating characteristic (ROC) curves were plotted to analyze the efficiency of oxidative stress combined with detection for assessing the cognitive impairment in OSAHS patients. Results. Snoring, mild to moderate OSAHS and severe OSAHS groups had significantly different snoring loudness, BMI, AHI, ODI, MoCA and MMSE scores, and levels of malondialdehyde (MDA), glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) (P<0.05). Cognitive impairment and normal cognitive groups had different BMI, GSH-Px, MDA and SOD levels, neuroglobin, hypoxia-inducible factor, AHI and lowest nocturnal oxygen saturation (P<0.05 or P<0.01). BMI, GSH-Px, MDA, SOD, neuroglobin, hypoxia-inducible factor, AHI and lowest nocturnal oxygen saturation were independent risk factors for cognitive impairment. The MoCA and MMSE scores of cognitive impairment had positive correlations with GSH-Px and SOD, but negative correlations with MDA (P<0.05). AUCs of GSH-Px, MDA, SOD and their combination for prediction were 0.670, 0.702, 0.705 and 0.836, respectively. Conclusion. Oxidative stress may be the biochemical basis of cognitive impairment in OSAHS patients.