O2 therapy, or oxygen therapy, involves the administration of supplemental oxygen to patients with hypoxaemia or respiratory distress to improve oxygen saturation and tissue oxygenation. It is crucial in clinical practice as it enhances patient outcomes in conditions like chronic obstructive pulmonary disease (COPD), pneumonia, and acute respiratory distress syndrome (ARDS). A key complication of O2 therapy is oxygen toxicity, which can lead to lung injury if high concentrations are used for prolonged periods.