  We explore the fermion oscillation in a degenerate environment. The direct
consequence is introducing a Pauli blocking factor $1 - f_i$, where $f_i$ is
the phase space distribution function, for each intermediate mass eigenstate
during propagation. It is then much easier for a state with larger existing
fraction or density to oscillate into other states with less degeneracy while
the reversed process is not enhanced. This can significantly modify the
oscillation behaviors. We apply this degenerate fermion oscillation to a
concrete scenario of neutron-antineutron oscillation in neutron star. It turns
out antineutrons receive a standing fraction to annihilate with the
environmental neutrons. The subsequent neutron star heating can put an
extremely stringent bound on the baryon number violating cross mass term
between neutron and antineutron.
