The chemical element with the symbol "O" is oxygen. It is a vital element for all known forms of life on Earth and is essential for respiration in aerobic organisms. Oxygen makes up about 21% of the Earth's atmosphere and is involved in various biochemical processes, including cellular respiration, which occurs in animals and plants. In addition to its role in supporting life, oxygen is also a critical component in combustion reactions, making it essential for burning fuels. Oxygen exists in diatomic form (O2) in the atmosphere and can also combine with other elements to form compounds, contributing significantly to the planet's chemistry.