The relative atomic mass, or atomic weight, of an element is the weighted average of the masses of the isotopes in the naturally occurring element relative to the mass of an atom of the carbon-12 isotope which is taken to be exactly 12.
For an element E with the naturally occurring isotopes aE, bE, cE, and with the respective abundances of A%, B%, C% etc,
the relative atomic mass (r.a.m.) = (A/100 x a) + (B/100 x b) + (C/100 x c)
In case Of Oxygen
Relative atomic mass = 16 x 99.762% + 17 x 0.038% + 18 x 0.200% = 16.008(here atomic masses are approximatley equal to the mass number, hence they are taken as 16, 17, 18 respectively)
Due to this natural occurance of 3 isotopes of oxygen we do not say it to be equal to 8+8
||% Natural Abundance