The study is designed to compare two approaches to oxygen therapy in adults requiring mechanical ventilation in the ICU in emergency circumstances. The study compares a conservative approach to oxygen therapy to a liberal approach. This 40 000 participant trial will be conducted in multiple countries and includes innovative design features. This includes linkage to identify enrolled patients in national ICU registries and response adaptive randomisation giving trial participants an increased chance of being assigned to the oxygen regimen associated with the lowest mortality risk while the trial is ongoing.