  We study the systematic decomposition of the dimension nine
neutron-antineutron oscillation operators at tree and one-loop levels. We
discuss the topologies' generation and the assignment of the chiral quarks. The
completed lists of the decompositions are provided. We furthermore show an
example that the neutron-antineutron oscillation occurs at one-loop level, with
the tiny neutrino mass being generated via the scotogenic model and proton
decay being evaded.
