The most common causes of severe head injuries are motor vehicle accidents and falls from a height that cause cardiac arrest. Cardiac arrest resulted in the patient being unconscious, no breathing, and no pulse. The incidence of cardiac arrest is mostly because the patient is hypoxic. Patients with severe head injuries are characterized by a Gasglow Coma Scale (GCS) of 8 or less. Patients with severe head injuries who often have decreased oxygen saturation should require a definitive airway to prevent patient death. This study aims to determine the level of oxygen in the blood on the incidence of cardiac arrest in patients with severe head injury. The research was conducted at Gambiran Hospital, Kediri City. Correlational research with cross sectional approaches. The sample of this research is 18 respondents. The sampling technique is total sampling. The statistical test of this research is Spearman's rho. The results of this study p value = 0.035 which means p < (α = 0.05). There is a relationship between the level of oxygen in the blood and the incidence of cardiac arrest in patients with severe head injury. Keywords: Blood Oxygen Level, Cardiac arrest, Severe Head Injury.