  La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in
sharp contrast with the metallic behavior of FeSe and other parent parent
compounds of iron superconductors. We demonstrate that the key ingredient that
makes La$_2$O$_3$Fe$_2$Se$_2$ a Mott insulator, rather than a correlated metal
dominated by the Hund's coupling is the enhanced crystal-field splitting,
accompanied by a smaller orbital-resolved kinetic energy. The strong deviation
from orbital degeneracy introduced by the crystal-field splitting also pushes
this materials close to an orbital-selective Mott transition. We predict that
either doping or uniaxial external pressure can drive the material into an
orbital-selective Mott state, where only one or few orbitals are metallized
while the others remain insulating.
