The goal of this randomised clinical trial is to compare the efficacy of exertional oxygen delivery by continous versus demand-based flow systems in patients with fibrotic Interstitial lung disease and chronic obstructive pulmonary disease . The main questions it aim to answer:

Is there a difference in lowest oxygen saturation? Is there a difference in saturation: time below <90%, pulse rate, distance and patient preferences ? It is a cross over study, so all patients will test both methods. Participants will perform 2 6-minute walk tests with the two different oxygen delivery systems.

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.

Inclusion Criteria:

• Age ≥ 18 years
• Verified diagnosis of fibrotic interstitial lung disease or chronic obstructive pulmonary disease
• Desaturation < 88% during 6-minute walk test or currently receiving ambulatory oxygen by continous oxygen
• Able to walk at least 50 meters during a 6-minute walk test
• Self-reported stable respiratory symptoms in the previous 2 weeks
• Cognitively able to understand and participate in the study
• Written informed consent

Exclusion Criteria:

• Any physical condition including paralysis, lower extremity pain, or other musculoskeletal problems limiting exercise performance
• Unstable heart condition or symptomatic stenotic valve disease
• Smoking during the previous 24 hours
• Pregnant women
• Anemia, Hb < 7.3 mmol/l (women) or < 8.3 mmol/l (men)
• Non-invasive ventilation

Full access to the protocol, participants dataset and statistical codes will be granted on reasonable request.

Full access to the protocol, participants dataset and statistical codes will be granted on reasonable request for up to 5 years after the study has ended.

Full access to the protocol, participants dataset and statistical codes will be granted on reasonable request.