  The unequal electronic occupation of localized orbitals (orbital
polarization), and associated lowering of symmetry and degeneracy, play an
important role in the properties of transition metal oxides. Here, we examine
systematically the underlying origin of orbital polarization, taking as
exemplar the 3$d$ manifold of Co$^{2+}$ in a variety of spin, orbital and
structural phases in the double perovskite La$_2$CoTiO$_6$ and the (001)
superlattice (LaCoO$_3$)$_1$+(LaTiO$_3$)$_1$ systems. Superlattices are of
specific interest due to the large experimentally observed orbital polarization
of their Co cations. Based on first principles calculations, we find that
robust and observable orbital polarization requires symmetry reduction through
the lattice structure; the role of local electronic interactions is to greatly
enhance the orbital polarization.
