  Neutron-antineutron oscillations are considered in the light of recently
proposed particle models, which claim to resolve the neutron lifetime anomaly,
indicating the existence of baryon violating $\Delta B=1$ interactions.
Possible constraints are derived coming from the non-observation of
neutron-antineutron oscillations, which can take place if the dark matter
particle produced in neutron decay happens to be a Majorana fermion. It is
shown that this can be realised in a simple MSSM extention where only the
baryon number violating term $u^cd^cd^c$ is included whilst all other R-parity
violating terms are prevented to avoid rapid proton decay. It is demostrated
how this scenario can be implemented in a string motivated GUT broken to MSSM
by fluxes.
