Upon ionisation to the cation, the bond lengths of $\mathrm{N}{}_2$ and $\mathrm{O}{}_2$ change respectively as

Ionisation of $\mathrm{N}{}_2$ removes an electron from the bonding $\sigma 2p_z$ orbital, so bond order falls from 3 to 2.5 and the bond lengthens. Ionisation of $\mathrm{O}{}_2$ removes an electron from the antibonding ${\pi }^{\ast }2p$ orbital, so bond order rises from 2 to 2.5 and the bond shortens.