This is a protocol for a Cochrane Review (Intervention). The objectives are as follows:. To assess the effects of apnoeic oxygenation before intubation in adults in the operating theatre, emergency department, intensive care unit (ICU), or pre-hospital settings. Primary objective. To assess the effect of oxygenation compared with no oxygenation during the apnoeic phase of intubation on length of hospital stay and peri-intubation critical desaturation, in adults in pre-hospital, emergency department, intensive care and operating theatre environments. Critical desaturation will be deemed to be a reduction in saturation of pulse oximetry (SpO2) to less than 80%. Secondary objectives. To assess the effect of oxygenation compared with no oxygenation during the apnoeic phase of intubation on the incidence of desaturation (SpO2 below 93%), lowest recorded SpO2, ICU length of stay, first-attempt intubation success rate and adverse event rate during intubation (e.g. cardiac arrhythmia), in adults in pre-hospital, emergency department, intensive care and operating theatre environments.