The objective of this study was to evaluate the supplemental oxygen use in hospital emergency departments (EDs) in Victoria. A prospective exploratory design was used. All patients attending the three-study EDs during the data-collection periods and who could give informed consent were eligible for inclusion. A total of 346 patients were recruited and the prevalence of oxygen administration was 48.3%. The most common reasons for oxygen administration were shortness of breath (40.1%), chest pain (34.7%) and hypoxaemia (29.9%). Patients who received oxygen were older (P < 0.001), had higher incidence of ambulance transport to ED (P < 0.001) and hospital admission (P < 0.001) and higher median respiratory (P < 0.001) and median heart rates (P = 0.008). Oxygen is a major component of emergency care. Patients who received oxygen were more likely to have clear evidence of physiological abnormalities; however, oxygen decision-making warrants more detailed investigation. European Journal of Emergency Medicine 19: 48-52 (C) 2012 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.