After a critical review of the different opinions, the author differentiates the indications for oxygen treatment of respiratory insufficiency to subjective and objective based on his own experience following a clinical observation of 100 patients with exacerbated chronic respiratory insufficiency, subjected to dosed, controlled and continuous oxygen therapy. The most essential objective indication for oxygen treatment is the manifestation of hypoxemia degree without or with CO2 retention. According to hypoxemia manifestation, determined by PaO2 level, the indications are absolute (with PaO2 under 40 mm Hg), urgent (with PaO2 under 30 mm Hg) and relative (with PaO2 over 50 mm Hg). The application of oxygen treatment with PaO2 over 700 mm Hg as well as the so called "preventive" O2 treatment (with no hypoxemia) is improper with a view to possible harmful side effects of oxygen. Oxygen treatment is contraindicated in all patients with unfavourable ventilation response to oxygen treatment. In case of non-effective O2 treatment (unfavourable ventilation response resp.) mechanical ventilation must be turned to as well as in all cases with patients in respiratory coma.