COVID-19 disease has led to many deaths worldwide and early detection of people at a high risk of severe forms of this disease would greatly help physicians. The presence of oxidative stress biomarkers may help identify high-risk individuals early in the course of the disease. 8-Hydroxy-2'-deoxyguanosine (8-OHdG) is a widely used biomarker for assessing endogenous oxidative DNA damage. In this study, the urinary 8-OHdG levels were determined in COVID-19 patients and COVID-19 patients with cancer by a dispersive liquid-liquid microextraction (DLLME) method using gas chromatography-mass spectrometry (GC-MS). The effects of essential parameters on the extraction method were investigated. The LOD and LOQ are equal to 1.7 nM and 5.1 nM, respectively. At varied concentrations of 8-OHdG (300, 400, and 600 nM), the relative standard deviation (RSD) ranged from 18.35% to 22.36%. The mean urinary 8-OHdG levels of cancer and COVID-19 patients were 13.20 ± 6.20 nmol mmol