The element represented by the letter "O" on the periodic table is oxygen. Oxygen is a vital chemical element with the atomic number 8. It is essential for life on Earth, as it is a key component of the air we breathe and is necessary for cellular respiration in animals and humans. Oxygen exists in two main forms: diatomic oxygen (O₂), which makes up about 21% of Earth's atmosphere, and ozone (O₃), which forms a layer that protects the planet from harmful ultraviolet radiation. It is highly reactive and forms compounds with most elements, playing a crucial role in many biological and chemical processes.