The chemical element with the symbol O is oxygen. It is a non-metal and is essential for life on Earth, as it is a critical component of cellular respiration in most living organisms. Oxygen is highly reactive and forms compounds with many elements. It is also a key part of water (H2O) and is involved in various chemical processes, including combustion. This element makes up about 21% of Earth's atmosphere and is also found in abundant quantities in the Earth's crust, where it is often combined with other elements.