Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetallic element and oxidizing agent that readily forms compounds (notably oxides) with most elements. Photosynthesis releases oxygen, and respiration consumes oxygen.xygen is the most abundant element by mass in the Earth's crust as part of oxide compounds such as silicon dioxide, making up almost half of the crust's mass. Oxygen is an important part of the atmosphere, and is necessary to sustain most terrestrial life as it is used in respiration.