Olive oil production, one of the main agro-industries in Mediterranean countries, generates significant amounts of olive mill wastewaters (OMWs), which represent a serious environmental problem, because of their high organic load, the acidic pH and the presence of recalcitrant and toxic substances such as phenolic and lipidic compounds (up to several grams per litre). In Italy, traditional disposal on the soil is the most common way to discharge OMWs. This work is aimed at investigating the efficiency and feasibility of AOPs and biological processes for OMW treatment. Trials have been carried out on wastewaters taken from one of the largest three-phase mills of Italy, located in Quarrata (Tuscany), as well as on synthetic solutions. Ozone and Fenton's reagents applied both on OMWs and on phenolic synthetic solutions guaranteed polyphenol removal efficiency up to 95%. Aerobic biological treatment was performed in a batch reactor filled with raw OMWs (pH =4.5, T=30 °C) without biomass inoculum. A biomass rich of fungi, developed after about 30 days, was able to biodegrade phenolic compounds reaching a removal efficiency of 70%. Pretreatment of OMWs by means of oxidation increased their biological treatability.