We investigate the influence of oxygen annealing on the room temperatureoptical and scintillation properties of CaWO$_4$ single crystals that are beingproduced for direct Dark Matter search experiments. The applied annealingprocedure reduces the absorption coefficient at the peak position of thescintillation spectrum ($\sim430$ nm) by a factor of $\sim6$ and leads to aneven larger reduction of the scattering coefficient. Furthermore, the annealinghas no significant influence on the \emph{intrinsic} light yield. An additionalabsorption occurring at $\sim400$ nm suggests the formation of O$^-$ holecenters. Light-yield measurements at room temperature where one crystal surfacewas mechanically roughened showed an increase of the \emph{measured} lightyield by $\sim40 %$ and an improvement of the energy resolution at 59.5 keV by$\sim12 %$ for the annealed crystal. We ascribe this result to the reduction ofthe absorption coefficient while the surface roughening is needed to compensatefor the also observed reduction of the scattering coefficient after annealing.
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