  Assuming the Lorentz and CPT invariances we show that neutron-antineutron
oscillation implies breaking of CP along with baryon number violation -- i.e.
two of Sakharov conditions for baryogenesis. The oscillation is produced by the
unique operator in the effective Hamiltonian. This operator mixing neutron and
antineutron preserves charge conjugation C and breaks P and T. External
magnetic field always leads to suppression of oscillations. Its presence does
not lead to any new operator mixing neutron and antineutron.
