The answer is 0.132606 amu.
The mass defect (Δm) is the total mass of protons and neutrons minus the mass of the atom.
mass of an oxygen-16 atom: 15.994914 amu
mass of a proton: 1.00728 amu
mass of a neutron: 1.00866 amu
An oxygen atom has 8 protons and 8 neutrons. Therefore:
Δm = 8 · 1.00728 + 8 · 1.00866 - 15.994914 = 8.05824 + 8.06928 - 15.994914 = 0.132606 amu
