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.
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