  Given an o-minimal structure ${\mathcal M}$ with a group operation, we show
that for a properly convex subset $U$, the theory of the expanded structure
${\mathcal M}'=({\mathcal M},U)$ has definable Skolem functions precisely when
${\mathcal M}'$ is valuational. As a corollary, we get an elementary proof that
the theory of any such ${\mathcal M}'$ does not satisfy definable choice.
