Oxygen, an essential molecule for life, is a colorless, tasteless, and odorless gas. It is important to put the right indication for oxygen therapy in the appropriate patient. Indications for oxygen therapy can be reviewed in two subsections: acute and long-term oxygen therapy. Oxygen can be administered via different systems after the need for oxygen support is determined in the acute or chronic settings. Degree of hypoxemia, patient's tolerance, and the underlying disease are the main criteria to be considered for the choice of delivery system. Oxygen can be delivered through low-flow systems (< 15 L/min), such as nasal cannula, simple face mask, mask with a reservoir, and diffuser mask, or high-flow systems, such as venturi mask and high-flow nasal cannula. The aim of this review is to describe oxygen delivery systems in patients with respiratory insufficiency.