The primary objective of this study was to evaluate the effects of the organic loading rate on the performance of an up-flow anaerobic sludge blanket (UASB) reactor treating olive mill effluent (OME), based on the following indicators: (i) chemical oxygen demand (COD) removal efficiency; and (ii) effluent variability (phenol, suspended solids, volatile fatty acids, and pH stability). The UASB reactor was operated under different operational conditions (OLRs between 0.45 and $32;kg;COD/m^3{cdot}day$) for 477 days. The results demonstrated that the UASB reactor could tolerate high influent COD concentrations. Removal efficiencies for the studied pollution parameters were found to be as follows: COD, $47{sim}92%$; total phenol, $34{sim}75%$; color, $6{sim}46%$; suspended solids, $34{sim}76%$. The levels of VFAs in the influent varied between 310 and 1,750 mg/L. Our measurements of the VFA levels indicated that some of the effluent COD could be attributed to VFAs (principally acetate, butyrate, iso-butyrate, and propionate) in the effluent, which occurred at levels between 345 and 2,420 mg/L. As the OLRs were increased, more VFAs were measured in the effluent. A COD removal efficiency of 90% could be achieved as long as OLR was kept at a level of less than $10;kg;COD/m^3{cdot}day$. However, a secondary treatment unit for polishing purposes is necessary to comply with discharge standards.