The element represented by the symbol "O" is oxygen. It is a crucial element for life on Earth, making up about 21% of the Earth's atmosphere. Oxygen is essential for respiration in animals and humans and is used by plants during photosynthesis to convert sunlight into energy. The element has an atomic number of 8 and is highly reactive, forming compounds with a wide range of other elements. Oxygen exists in nature primarily in the form of diatomic molecules (O2) and is vital for combustion processes. Its discovery is attributed to several scientists, including Joseph Priestley and Carl Wilhelm Scheele in the 18th century.