Oxygen has long been considered a vital and potentially life-saving component of emergency care. Given this, there is widespread and liberal use of supplemental oxygen in hospitals across the United States and throughout the world. Recent research, however, delineates serious deleterious effects at the cellular level, inducing damage to the cardiovascular system, the central nervous system, the pulmonary system, and beyond. A scoping review was conducted to identify and synthesize available research data as it pertains to the clinical effects of hyperoxia in critically ill adult patients in acute care settings. We searched PubMed, MEDLINE, CINAHL, and Scopus databases. We also reviewed the reference lists of included publications. The selection of relevant articles was conducted by 2 researchers at 2 levels of screening. The review identified 30 studies, of which 5 were randomized controlled trials, 2 were prospective cohort studies, and 23 were retrospective cohort studies. A descriptive analysis of study results was performed. Current evidence suggests an association between hyperoxia and increased mortality after cardiac arrest, stroke, and traumatic brain injury, as well as in the setting of sepsis, although there is insufficient evidence to conclude concretely that hyperoxia effects clinical outcomes. As such, there exists a need for additional large-scale randomized controlled trials with well-defined parameters for the evaluation of clinical outcomes. Until the completion of such trials, titration of supplemental O to normoxia is advised to avoid the negative effects of both hyperoxia and hypoxia in acutely ill adult patients.Copyright © 2020 by Daedalus Enterprises.