In this study, individual and combined effects of various advanced oxidation processes (individual experiments: UV, O-3, US, H2O2; combined experiments: UV/US, UV/O-3, US/O-3, UV/H2O2, US/H2O2, O-3/H2O2, UV/US/O-3, UV/US/H2O2, UV/ O-3/H2O2, US/ O-3/H2O2 and UV/US/O-3/H2O2) as a pre-treatment option for olive mill effluent (OME), which is known to contain a significant amount of inhibitory compounds (e.g., phenolics and tannins), were comparatively evaluated in order to enhance the anaerobic degradation of OME. Following the pretreatment with the AOPs given above, Biochemical Methane Potential (BMP) assay was employed in order to monitor and comparatively evaluate any increase in biogas production as an indicator of improvement in anaerobic biological degradation before and after pretreatment. It was observed that pretreatment provided significant enhancement in biodegradation of OME. Compared to the results obtained with raw OME, all pretreated samples, except for a few cases, showed 2-6 fold enhancement in anaerobic degradation of the OME samples studied. Amongst AOPs applied, UV, UV/H2O2/US, UV/O-3, and UV/US/O-3/H2O2 were the best in terms of enhancement in anaerobic degradation. Both single and combined pretreatment processes resulted in about 50% COD removal. In addition, total phenol degradation between 80-98% was achieved by pretreatment with UV/O-3/H2O2, UV/US/O-3/H2O2, UV/O-3 and UV alone.