Obstructive sleep apnoea/hypopnoea syndrome (OSAHS) is a highly prevalent breathing disorder in sleep that is an independent risk factor for cardiovascular morbidity and mortality. A large body of evidence, including clinical studies and cell culture and animal models utilising intermittent hypoxia, delineates the central role of oxidative stress in OSAHS as well as in conditions and comorbidities that aggregate with it. Intermittent hypoxia, the hallmark of OSAHS, is implicated in promoting the formation of reactive oxygen species (ROS) and inducing oxidative stress. The ramifications of increased ROS formation are pivotal. ROS can damage biomolecules, alter cellular functions and function as signalling molecules in physiological as well as in pathophysiological conditions. Consequently, they promote inflammation, endothelial dysfunction and cardiovascular morbidity. Oxidative stress is also a crucial component in obesity, sympathetic activation and metabolic disorders such as hypertension, dyslipidaemia and type 2 diabetes/insulin resistance, which aggregate with OSAHS. These conditions and comorbidities could result directly from the oxidative stress that is characteristic of OSAHS or could develop independently. Hence, oxidative stress represents the common underlying link in OSAHS and the conditions and comorbidities that aggregate with it.