  The phenomenology of neutrino oscillations is extended to the neutron-
antineutron oscillations predicted by the grand unified theories. There are six
parameters in general describing the one-flavor oscillations: two real Majorana
masses (of opposite signs), three Euler's angles, and a phase multiplier "chi".
The sum of the Majorana masses, treated as a small parity- violating parameter
"epsilon", induces the nuclear decays (A,Z) -> (A-2,Z) and determines the
oscillation rate of neutrons in the vacuum. We derive a constraint to a small
parity-conserving parameter "epsilon-prime" representing a combination of two
Euler's angles. This parameter gives no significant contribution to the neutron
oscillation rate in the vacuum, while contributes to the nuclear decay rate.
The nuclear stability implies "epsilon-prime" < 1/year. Absence of the vacuum
neutron oscillations together with existence of the nuclear decays (A,Z) ->
(A-2,Z) would become simplest manifestation of the "epsilon-prime" =/ 0.
