  We present an experimental study on the rotational inelastic scattering of OH
($X^2\Pi_{3/2}, J=3/2, f$) radicals with He and D$_2$ at collision energies
between 100 and 500 cm$^{-1}$ in a crossed beam experiment. The OH radicals are
state selected and velocity tuned using a Stark decelerator. Relative
parity-resolved state-to-state inelastic scattering cross sections are
accurately determined. These experiments complement recent low-energy collision
studies between trapped OH radicals and beams of He and D$_2$ that are
sensitive to the total (elastic and inelastic) cross sections (Sawyer \emph{et
al.}, \emph{Phys. Rev. Lett.} \textbf{2008}, \emph{101}, 203203), but for which
the measured cross sections could not be reproduced by theoretical calculations
(Pavlovic \emph{et al.}, \emph{J. Phys. Chem. A} \textbf{2009}, \emph{113},
14670). For the OH-He system, our experiments validate the inelastic cross
sections determined from rigorous quantum calculations.
