What is atomic mass ?

Atomic mass refers to the mass of an atom. It depicts how many times an atom of an element is heavier than one-twelth (1/12^th) the mass of one atom of carbon-12 of mass of one carbon atom. The relative atomic masses of all elements have been established with reverence to an atom of carbon-12. It is measured in unit called amu (atomic mass unit). 1/12 of the mass of one atom of carbon is termed as relative atomic mass. It doesnot possess a unit.

Mathematically,

mass of one atom of an element = atomic mass X (1/12th) of the mass of one atom of carbon. 

Or

atomic mass = mass of one atom of an element / (1/12th) of the mass of one atom of carbon.

Easy way to calculate atomic mass of oxygen atom.

The Proton number Z = 8
The Neutron number N = 8
The atomic mass of the oxygen atom is given by 15.9994 amu i.e. nearly 16.