  We discuss the interplay between lepton asymmetry L and neutrino oscillations
in the early Universe. Neutrino oscillations may suppress or enhance previously
existing L. On the other hand L is capable to suppress or enhance neutrino
oscillations. The mechanism of L enhancement in MSW resonant neutrino
oscillations in the early Universe is numerically analyzed. L cosmological
effects through neutrino oscillations are discussed.
  We discuss how L may change the cosmological BBN constraints on neutrino and
show that BBN model with electron-sterile neutrino oscillations is extremely
sensitive to L - it allows to obtain the most stringent constraints on L value.
We discuss also the cosmological role of active-sterile neutrino mixing and L
in connection with the indications about additional relativistic density in the
early Universe, pointed out by BBN, CMB and LSS data and the analysis of global
neutrino data.
