The chemical element with the symbol 'O' is oxygen. It is essential for life on Earth, serving as a critical component of the air we breathe and a vital player in cellular respiration. Oxygen is a colorless, odorless gas at room temperature and makes up about 21% of the Earth's atmosphere. It is also highly reactive, readily forming compounds with other elements, such as water (H₂O) and carbon dioxide (CO₂). In addition to its significance in biological processes, oxygen is involved in combustion and various industrial applications, such as steelmaking and water treatment.