The aim of the present study was to evaluate process variables and intermediate outcomes involved in long-term oxygen therapy (LTOT) by concentrator with the purpose of identifying which of those factors would be the most influential in the final health outcome of the therapy. A cross-sectional survey was carried out on a random sample of 111 patients receiving LTOT by concentrator in Catalonia (Spain). Patients were interviewed and assessed at home by a trained physician, and the variables collected were arterial oxygen saturation, performance of the concentrators, and patient compliance. Sixty-two patients participated in the study. Overall, LTOT was appropriately prescribed in 36 patients, of whom only 29 were able to correct their level of hypoxaemia. Patient compliance with treatment was considered adequate in 19 of those 29 patients. Thus, only 19 of 62 patients (31%) fulfilled those criteria needed to achieve the expected clinical benefits. Strategies for improving the effectiveness of medical interventions or technologies ought to consider those factors of the therapeutic process which might influence the expected health outcomes in a specific health-care context.