  Recently, superconductivity has been discovered in rock-salt structured
binary lanthanum monoxide LaO through state-of-the-art oxide thin-film epitaxy.
In this work, we reveal that the normal state of superconducting LaO is a $Z_2$
nontrivial topological metal, where the Dirac point protected by the crystal
symmetry is located around the Fermi energy. By analysing the orbital
characteristics, we show that the nature of the topological band structure of
LaO originates from the intra-atomic transition from the outer shell La 5$d$ to
the inner shell 4$f$ orbitals driven by the strong octahedral crystal-field.
Furthermore, the appearance of novel surface states unambiguously demonstrates
the topological signature of LaO superconductor. Our theoretical findings not
only shed new light into the understanding of the exotic quantum behaviors in
LaO superconductor with intimate correlation between 4$f$ and 5$d$ orbitals in
La, but also provide an exciting platform to explore the interplay between
nontrivial topology and superconductivity.
