The element represented by the symbol "O" is oxygen. Oxygen is a colorless, odorless gas essential for life on Earth and is the most abundant element in the Earth's crust. It makes up about 21% of the Earth's atmosphere and is crucial for respiration in living organisms, serving as a fundamental part of cellular processes. Oxygen forms compounds with almost all elements and is involved in combustion and oxidation reactions. In its diatomic form (O2), oxygen supports life and is necessary for the processes of photosynthesis in plants and breathing in animals.