  We discuss the potential of SNO to constraint the four-neutrino mixing
schemes favoured by the results of all neutrino oscillations experiments. These
schemes allow simultaneous transitions of solar $\nu_e's$ into active
$\nu_\mu$'s, $\nu_\tau$'s and sterile $\nu_s$ controlled by the additional
parameter $\cos^2(\vartheta_{23}) \cos^2(\vartheta_{24})$ and they contain as
limiting cases the pure $\nu_e$-active and $\nu_e$-sterile neutrino
oscillations. We first obtain the solutions allowed by the existing data in the
framework of the BP00 standard solar model and quantify the corresponding
predictions for the CC and the NC/CC event ratios at SNO for the different
allowed regions as a function of the active-sterile admixture. Our results show
that some information on the value of $\cos^2(\vartheta_{23})
\cos^2(\vartheta_{24})$ can be obtained by the first SNO measurement of the CC
ratio, while considerable improvement on the knowledge of this mixing will be
achievable after the measurement of the NC/CC ratio.
