The chapter discusses the role of oxygen radicals (OFR) in DNA damage and cancer incidence. OFR, which are intermediates of oxygen reduction, can damage DNA by attacking both the bases and the deoxyribose backbone. These radicals can also react with other cellular components, such as lipids, to form reactive species that further damage DNA. The most extensively studied lesion is 8-OH-dG, which is mutagenic and a potential biomarker of carcinogenesis. Antioxidants, including non-enzymatic antioxidants like vitamin C and E, carotenoids, and antioxidant enzymes, help counteract the effects of OFR. However, under certain conditions, some antioxidants can exhibit pro-oxidant effects. Clinical studies have shown that increasing antioxidant intake does not significantly reduce cancer incidence. Instead, reducing endogenous and exogenous sources of oxidative stress is recommended. The chapter also delves into the electronic structure of oxygen radicals, their sources, and their properties, highlighting their importance in understanding the mechanisms of DNA damage and cancer development.The chapter discusses the role of oxygen radicals (OFR) in DNA damage and cancer incidence. OFR, which are intermediates of oxygen reduction, can damage DNA by attacking both the bases and the deoxyribose backbone. These radicals can also react with other cellular components, such as lipids, to form reactive species that further damage DNA. The most extensively studied lesion is 8-OH-dG, which is mutagenic and a potential biomarker of carcinogenesis. Antioxidants, including non-enzymatic antioxidants like vitamin C and E, carotenoids, and antioxidant enzymes, help counteract the effects of OFR. However, under certain conditions, some antioxidants can exhibit pro-oxidant effects. Clinical studies have shown that increasing antioxidant intake does not significantly reduce cancer incidence. Instead, reducing endogenous and exogenous sources of oxidative stress is recommended. The chapter also delves into the electronic structure of oxygen radicals, their sources, and their properties, highlighting their importance in understanding the mechanisms of DNA damage and cancer development.