Rate of effusion $\propto \frac{1}{\sqrt{M}}$
$[M=$ Molecular mass $]$
as molecular mass of $O _{2}=32 \,g$
molecular mass of $N _{2}=28 \,g$
molecular mass of $O _{2}$ is more than $N _{2}$
$\therefore $ rate of effusion of $O _{2}$ is less than that of $N _{2}$
$ \because r \propto \frac{1}{\sqrt{m}}$
Molecular size of nitrogen is greater than oxygen as size decrease in a period from left to right