Treatability of Olive Mill Wastewater (OMW) by Pulsed High Voltage Discharge (PHVD) reactor using pure oxygen or air was investigated. It was shown that OMW was effectively treated through the combined reactions of the hydroxyl radicals, ultraviyole and ozone formed by applying pulsed electrical discharges in the presence of pure oxygen or air. The results show that PHVD using oxygen displays better performance than PHVD using air. PHVD process using pure oxygen removed 75 % of chemical oxygen demand (COD), 61 % of total organic carbone (TOC), 71 % of phenol and 76 % of color in OMW in 60 min under optimum conditions (pH = 10, temperature = 13 degrees C, conductivity = 30 mS/cm, voltage = 24 kV, flow rate = 7.5 mL/min.). Oxidation kinetic results showed that the both two oxidation processes behave as a pseudo-second-order reaction with respect to COD and phenol removal. When they are compared in terms of the operational costs, the operational cost of PHVD reactor using Air was lower than the cost of PHVD reactor using oxygen.