The study design is a single center, randomized, open-label cross-over exploratory comparative study to investigate the efficacy of two different oxygen delivery systems. Patients with chronic obstructive Pulmonary disease or fibrotic Interstitial Lung Disease that during a six-minute walk test can walk at least 50 m and desaturate below 88%, can be included in the study. The participants are performing 2 6-minute walk tests, randomized to perform the test with oxygen bottles or portable concentrators first. Primary endpoint is the difference in the lowest oxygen saturation between the two systems. Secondary endpoints are amongst others: difference in percentage of time and number of minutes when oxygen falls below 90%, mean and maximum pulse rate, distance and time taken to recover during the 6-minute walk test and scores from questionnaires. After 3-6 months, semistructured interviews will be done to record participants' experiences of ambulatory oxygen therapy.
