The chemical element with the symbol 'O' is oxygen. Oxygen is essential for life as it is a critical component of cellular respiration, allowing organisms to convert glucose and nutrients into energy. It comprises about 21% of Earth's atmosphere and is vital for most living beings, including humans, who rely on it for survival. In addition to its role in respiration, oxygen is also involved in various chemical reactions and processes, such as combustion and oxidation. Its highly reactive nature enables it to form compounds with almost all other elements, making it a fundamental element in chemistry and biology.