  This article provides quasi-optimal a priori error estimates for an optimal
control problem constrained by an elliptic obstacle problem where the finite
element discretization is carried out using the symmetric interior penalty
discontinuous Galerkin method. The main proofs are based on the improved
$L^2$-error estimates for the obstacle problem, the discrete maximum principle,
and a well-known quadratic growth property. The standard (restrictive)
assumptions on mesh are not assumed here.
