More than 20,000 New Zealanders (and 20M people globally) are admitted to intensive care units (ICU) annually. ICU patients are the sickest patients in the hospital; most require life support (invasive mechanical ventilation), and many die from their critical illness. Supplemental oxygen is universally administered to patients who require life-support but the correct dose of oxygen is currently unknown. We are undertaking a 1000-participant randomised controlled trial to see whether avoiding abnormally high or low levels of oxygen improves outcomes compared to standard oxygen therapy in ICU patients who require life support. The second part of this study aims to assess the way oxygen is used in ICUs across New Zealand, Australia (ANZ) and the United Kingdom and whether this changes in ICUs participating in the above study, compared to ICUs who don’t participate. It also investigates the attitudes of health professionals towards oxygen therapy in ANZ.