Oxygen-16 (symbol: 16O or 168O) is a nuclide. It is a stable isotope of oxygen, with 8 neutrons and 8 protons in its nucleus, and when not ionized, 8 electrons orbiting the nucleus. The atomic mass of oxygen-16 is 15.99491461956 Da. It is the most abundant isotope of oxygen and accounts for 99.757% of oxygen's natural abundance.

The relative and absolute abundances of oxygen-16 are high because it is a principal product of stellar evolution and because it is a primordial isotope, meaning it can be made by stars that were initially made exclusively of hydrogen.

Most oxygen-16 is synthesized at the end of the helium fusion process in stars; the triple-alpha process creates carbon-12, which captures an additional helium-4 to make oxygen-16. It is also created by the neon-burning process.

Oxygen-16 is doubly magic.

Solid samples (organic and inorganic) for oxygen-16 studies are usually stored in silver cups and measured with pyrolysis and mass spectrometry. Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.

Historically, one atomic mass unit was defined as one sixteenth of the mass of an oxygen-16 atom, but has been redefined as one twelfth of the mass of a carbon-12 atom, with the name unified atomic mass unit or dalton.