A toxic effect in a living organism caused by a species of oxygen. Oxygen has two aspects, one benign and the other malignant. Those organisms that avail themselves of the enormous metabolic advantages provided by dioxygen (O₂) must defend themselves against its toxicity. The complete reduction of one molecule of O₂ to two of water (H₂O) requires four electrons; therefore, intermediates must be encountered during the reduction of O₂ by the univalent pathway. The intermediates of O₂ reduction, in the order of their production are the superoxide radical O₂⁻), hydrogen peroxide (H₂O₂), and the hydroxyl radical (HO·).