Radiative recombination processes provide valuable information about exciton dynamics and allow detection of defects in rubrene crystals. We demonstrate that the photoluminescence spectra of crystalline rubrene reflect exciton dissociation through oxygen-related defects in addition to the direct exciton recombination. The defect-assisted exciton dissociation results in a well-defined photoluminescence band. These defects play an important role in charge transport. Dark- and photo-conductivity is higher in rubrene crystals with a large density of the defects. The observations strongly suggest that the oxygen-related defect forms a bandgap state and acts as an acceptor center in crystalline rubrene.