  Ordinary-sterile neutrino oscillations can generate significant neutrino
asymmetry in the early Universe. In this paper we extend this work by computing
the evolution of neutrino asymmetries and light element abundances during the
Big Bang Nucleosynthesis (BBN) epoch. We show that a significant
electron-neutrino asymmetry can be generated in a way that is approximately
independent of the oscillation parameters $\delta m^2$ and $\sin^2 2\theta$ for
a range of parameters in an interesting class of models. The numerical value of
the asymmetry leads to the prediction that the effective number of neutrino
flavours during BBN is either about 2.5 or 3.4 depending on the sign of the
asymmetry. Interestingly, one class of primordial deuterium abundance data
favours an effective number of neutrino flavours during the epoch of BBN of
less than 3.
