Oxygen is considered as a double edge sword in mechanical ventilation where it saves the person from mortality and morbidity on one hand and on the other it can cause damage if there is an excess (oxygen toxicity) or if there is lack of oxygen (hypoxia). Oxygen toxicity also known hyperoxia is one of the complication of mechanical ventilation where high oxygen levels cause damage to lungs by various oxidative stress mechanisms. It results when patient receives more than normal amount of FiO2 for prolonged period of time. High oxygen levels in the body causes activation of inflammatory response and in turn formation of reactive oxygen species. These reactive oxygen species produced in turn cause damage lung epithelium, inactivate lung surfactant, cause pulmonary edema, thickening, fibrosis and collapse of lung alveoli.