The approximate mass of 1 atom of $O^{16}$ in gms is $\frac{16}{{\mathrm{N}}_{\mathrm{A}}}$.
 1 atom of$O^{16}$ weighs $16 \mathrm{a}.\mathrm{m}.\mathrm{u}$ and $1 \mathrm{a}.\mathrm{m}.\mathrm{u}$ $=\frac{{\mathrm{N}}_{\mathrm{A}}}{1}$ gm
 

Since 1 atom of$O^{16}$weighs $16 \mathrm{a}.\mathrm{m}.\mathrm{u}$ and $1 \mathrm{a}.\mathrm{m}.\mathrm{u}$.equals $\frac{1}{{\mathrm{N}}_{\mathrm{A}}}$gm, the approximate mass of 1 atom of $O^{16}$ in gms is $\frac{16}{{\mathrm{N}}_{\mathrm{A}}}$.

Both Assertion and Reason are correct and reason is the correct explanation of Assertion.