The percentage of oxygen in NaOH is

There is only one O atom in NaOH. Molar mass of NaOH is 40 g/mol and atomic mass of O is 16 g/mol.

 $\% \mathrm{of} \mathrm{O} \mathrm{in} \mathrm{NaOH} =\frac{16}{40}\times 100=40\mathrm{\%}$