Molecular oxygen (O2), a colorless, odorless gas at room temperature.
(medicine) A mixture of oxygen and other gases, administered to a patient to help him or her to breathe.
(countable) An atom of this element.
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Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light.