Open heart surgery using cardiopulmonary bypass is a common procedure, with more than 3000 operations each year in New Zealand and over 1 million worldwide. Damage to organs, including kidneys, liver and heart is common after heart surgery and some of the damage may be caused by extra oxygen given on cardiopulmonary bypass. This study will randomise patients to receive normal levels of oxygen or the traditional high levels during heart surgery. Endpoints include a number of blood tests that are very sensitive for detecting damage to the kidney, liver, heart and other organs. In total 298 patients having heart surgery at Auckland City Hospital will be included in this study over a two-year period.