The symbol for the element oxygen is O. Oxygen is a vital chemical element and is essential for life on Earth. It is a colorless, odorless gas at room temperature and makes up about 21% of the earth's atmosphere. The atomic number of oxygen is 8, which means it has eight protons in its nucleus. Oxygen plays a crucial role in respiration for many living organisms, and it is also involved in combustion and various chemical reactions. Its presence in water (H2O) is essential for all known forms of life.