The atomic size decreases on moving from left to right in a period as the number of valence electrons in the same shell increases due to which the effective nuclear charge increases. Thus, size of O is less than N and also size of S is less than P. Now, on moving down in a group atomic size increases because each time one new shell is added and electrons in the outermost shell move away from the nucleus. Thus, size of O<S and N<P. Thus, the overall order is O<N<S<P.