Diabetes mellitus and breast cancer are two important health problems. Type 2 diabetes (T2DM) and obesity are closely linked with both being associated with breast cancer. Despite abundant epidemiological data, there is no definitive evidence regarding the mechanisms responsible for this association. The proposed mechanisms by which diabetes affects breast cancer risk and prognosis are the same as the mechanisms hypothesised for the contribution of obesity to breast cancer risk. The obesity-induced inflammation promoted by adipose tissue dysfunction is a key feature, which is thought to be an important link between obesity and cancer. Inflammation induces an increase in free radicals and subsequently promotes oxidative stress, which may create a microenvironment favourable to the tumor development in obese persons. Oxidative stress is also proposed as the link between obesity and diabetes mellitus. Therefore, obesity-related oxidative stress could be a direct cause of neoplastic transformation associated with obesity and T2DM in breast cancer cells. This review is focused on the role of obesity-related oxidative stress in the context of chronic inflammation, on the time of breast cancer onset and progression, which provide targets for preventive and therapeutic strategies in the fields of diabetes and obesity-related breast cancer.