Chemistry. Chemical element with the symbol O and atomic number 8. The colorless, odorless and tasteless gas is very common and ubiquitous, it comprises about 20% of the atmosphere of Earth (as molecular oxygen, O2, or ozone, O2) and about 87% of the oceans (as water, H2O). Oxygen is essential for all organisms, except anaerobic bacteria. Common physiological compounds are carbon dioxide, CO2, carbon monoxide CO, hydroxide OH, and peroxide such as H2O2.