The atomic mass of an oxygen atom is equal to 16 atomic mass units (u) or unified atomic mass units (u). This value represents the average mass of an oxygen atom, taking into account the various isotopes of oxygen and their relative abundance in nature. The most common isotope of oxygen is oxygen-16, which has an exact atomic mass of 16 atomic mass units. Other isotopes, such as oxygen-17 and oxygen-18, have slightly higher atomic masses due to the presence of additional neutrons in their nuclei. However, the average atomic mass of oxygen, considering the abundance of these isotopes, is approximately 16 u.