The average atomic mass of oxygen is 15.9994 amu. the atomic masses of its three isotopes are as follows: o-16: 15.995 amu; o-17: 16.999 amu; o-18: 17.999 amu which isotope is most likely to have the greatest abundance in nature? o–16 o–17 o–18 all three isotopes may be equally abundant

The average atomic mass of oxygen is the atomic mass with respect to 1/12th the mass of a C-12 atom, related to natural abundance. 
amu - atomic mass units 
O-16 - 15.995 amu
O-17 - 16.999 amu
O-18 - 17.999 amu
the average atomic mass is 15.9994 amu
average atomic mass = ∑atomic mass of each isotope * relative abundance percentage
in other words the average atomic mass is closer to the atomic mass of the most abundant isotope. In this case 15.9994 is closest to 15.995 amu which is the atomic mass of O-16.
This means that O-16 is the most abundant isotope.

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