  We develop a new method to describe electromagnetic observables of open-shell
nuclei with two nucleons outside a closed shell. This approach combines the
equation-of-motion coupled-cluster method for such systems and the Lorentz
integral transform technique, expanding the applicability of coupled-cluster
theory for these properties beyond closed-shell nuclei. To validate this new
approach, we compute the non-energy-weighted dipole sum rule and the dipole
polarizability of $^{16,24}$O in both the closed-shell and the new
equation-of-motion coupled-cluster frameworks, finding agreement within error
bars. We then analyze the evolution of the dipole polarizability along the
oxygen and calcium isotopic chains. Our predictions agree well with available
experimental data and other available theoretical calculations for the
closed-shell $^{16,22}$O and the open-shell $^{18}$O. In the calcium isotopes,
we observe that our dipole polarizability predictions for open-shell nuclei are
lower than those of closed-shell nuclei. Our predictions for $^{24}$O and
$^{54,56}$Ca will motivate future experimental studies at the dripline.
