  In grand unified theories the flavor mixing angles of leptons are related to
those of quarks. In this paper we study SO(10) theories with a precise group
theoretic relation at the GUT scale for mixing between the heaviest two
generations: $\thetuto = \kappa \abs{\vcbo}$. A comprehensive operator search
yields all possible cases where $\kappa$ is a group theory Clebsch. The
resulting predictions for $\nu_\mu \nu_\tau$ oscillations are scaled from the
grand to the weak scales. We find that all but one of the models which have
such a relationship between $\thetuto$ and $\vcbo$, and are not already
excluded, will be probed by the CHORUS and NOMAD experiments. A more precise
mixing measurement, for example by the proposed P803 experiment, could
distinguish between the models.
