What is the atomic number for the oxygen atom?

The mass number is the sum of neutrons and protons in the atomic nucleus and is different for each isotope of any element.

For example the isotope oxygen-16 has the mass number 16.

The atomic weight of oxygen (not Atomic Mass) is [15,99903; 15,99977] after the IUPAC Tables - 2009.

The atomic number of oxygen is 8.