The chemical element with the symbol O is oxygen. It is a colorless, odorless gas that is essential for life on Earth. Oxygen is a vital component of cellular respiration for most living organisms, allowing them to convert food into usable energy. It constitutes about 21% of the Earth's atmosphere and is the most abundant element in the universe by mass. Oxygen plays a crucial role in combustion processes and various chemical reactions. Additionally, it is essential for the formation of water and a key element in many biological compounds, contributing significantly to Earth's ecosystems.