Abstract Background Prehospital high concentration oxygen therapy leads to worse clinical outcomes in patients presenting with acute exacerbations of chronic obstructive pulmonary disease (AECOPD). Less is known about the risks of hypoxaemia despite oxygen treatment. Current respiratory and ambulance guidelines recommend titration of supplemental oxygen to a target oxygen saturation range of 88%–92%. Aim To explore the association between PaO2 and risk of serious adverse clinical outcomes in AECOPD. Methods A retrospective review of consecutive patients presenting via ambulance to the Wellington Regional Hospital Emergency Department with AECOPD between June 2005 and January 2008. Patients with an arterial blood gas taken within 4 h of triage were included in the study and were categorised as hypoxaemic (PaO2<60 mm Hg), normoxaemic (PaO2 60–100 mm Hg) or hyperoxaemic (PaO2>100 mm Hg). Serious adverse outcome was defined as a composite of hypercapnic respiratory failure, assisted ventilation or inpatient death. Multivariate logistic regression analysis examined the association between PaO2 category and the composite outcome. Results Of the 680 patients presenting with AECOPD in the review period, 254 presentations in 180 patients had data suitable for analysis. Hyperoxaemia occurred in 61/254 (24%) presentations and was strongly associated with serious adverse outcome compared with normoxaemia (OR 9.17, 95% CI 4.08 to 20.6). Hypoxaemia was also associated with an increased risk of serious adverse outcome compared with normoxaemia (OR 2.16, 95% CI 1.11 to 4.20). Compared with the recommended target oxygen saturation range of 88%–92%, the risk of a serious adverse outcome was increased in both the <88% group (OR 2.0, 95% CI 1.03 to 3.80) and the >96% group (OR 2.37, 95% CI 1.34 to 4.20). Conclusions In patients presenting via ambulance to the Emergency Department with AECOPD, serious adverse clinical outcomes are associated with both hypoxaemia and hyperoxaemia. These data provide further support for the principle of titrating supplemental oxygen therapy to target oxygen saturations.