The study subjects will be patients with a confirmed diagnosis of COPD who have been accepted for PR and who are not hypoxemic at rest, but have exercise-induced desaturation (defined as a fall in oxygen saturations (SaO2) ≥4% to at least <90%, or any fall to a SaO2 <90%) and demonstrate positive improvement with use of ambulatory oxygen as per the British Thoracic Society 2015 Oxygen guidelines. It is standard practice for patients to have this assessment prior to commencing PR and as part of the assessment the flow rate required to increase the SaO2 to ≥90% during exercise will be determined. Patients will be recruited from PR programs at imperial college health care trust hospitals.