Due to its small atomic number and stability, there are many signatures of oxygen in the Universe. At optical wavelengths the forbidden oxygen III emission line is prevalent in many galactic spectra, whilst at radio wavelengths the OH megamasers are visible at great distances from the Sun in external galaxies.
At standard temperatures and pressures on Earth, pure oxygen forms the diatomic molecule O2. On Earth oxygen is essential for life. We breathe in O2 and expel CO2. Water is another example of a compound (H2O) that also contains oxygen. Oxygen is a very reactive element and quite corrosive.