Re: Why is the atomic mass of oxygen 15.999?

That is a very good question, David. The answer is that the isotopes of various elements have masses 
that are close to, but not exactly the same as the mass number. 

Strangely, if you have more than one heavy particle (proton or neutron) in an atomic nucleus, the mass 
of the nucleus is not equal to the sum of the masses of the particles in it! There is a fairly complicated 
reason for that, related to the special theory of relativity, and Einstein's famous equation E = m * c^2

An O-16 atom has relative mass 15.99491; O-17 is 16.99913; O-18 is 17.99916.

(Data from NIST tables)

As you would expect, the atomic mass of oxygen, the weighted natural mix of isotopes, is just 
fractionally greater than that of O-16, even though it is less than 16.0000.