The mass number of oxygen is 16.
This value is determined by the sum of the number of protons and neutrons in the nucleus of an oxygen atom. Oxygen has 8 protons, which is what defines it as oxygen, and it typically has 8 neutrons. Therefore, when you add these together (8 protons + 8 neutrons), you get a mass number of 16. This mass number is significant because it helps distinguish between various isotopes of an element. For example, while most oxygen found in nature is oxygen-16, there are rare isotopes like oxygen-17 and oxygen-18, which have different numbers of neutrons.