  The maximum overproduction of helium-4 in cosmological nucleosynthesis with
active--sterile neutrino oscillations, nu_e -> nu_s, efficient after decoupling
of electron neutrino, is analyzed. The kinetic effects on primordial
nucleosynthesis due to neutrino spectrum distortion, caused by oscillations,
are precisely taken into account.
  The maximum overproduction of primordial He-4 as a function of oscillation
parameters is obtained from the analysis of the kinetics of the nucleons and
the oscillating neutrinos, for the full range of parameters of the discussed
oscillation model. A maximum relative increase of He-4, up to 14% for
non-resonant oscillations and up to 32% for resonant ones is registered.
Cosmological constraints on oscillation parameters are also discussed.
