Oxygen treatment is commonly used in clinical practice. Although this treatment was taught during medical education under different titles, it is observed that doctors do not administer oxygen treatment in adequate periods and doses. The possible cause of this may be that oxygen is not considered as a drug. The results of inadequate dose and insufficient monitoring in oxygen treatment would be serious. On the other hand, failure to correct hypoxaemia fearing from hypoventilation and carbon dioxide retention is not acceptable. For a safe oxygen treatment, doctor must know its indications, oxygen delivery systems, flow rates and monitoring. The aim of this review is to refresh our knowledge about when, how and how much to start oxygen treatment and how to monitor it.