The broad objective of this study is to test the association between hyperoxia exposure after resuscitation from cardiac arrest and outcome. After obtaining written informed consent subjects enrolled in REOX II will undergo a rapid faction of inspired oxygen (FiO2) optimization protocol to prevent exposure to hyperoxia. We will compare outcomes between subjects enrolled in REOX I (observational study only) and REOX II (intervention: rapid FiO2 optimization protocol). Our overarching hypothesis is that exposure to hyperoxia after return of spontaneous circulation (ROSC) is associated with increased oxidative stress and worsened neurological and cognitive outcomes.