Fresh olive oil mill wastewaters (OMW) were collected in northern Morocco from industrial mills operating with three extraction systems (dual [C2] and triple phase [C3] centrifugation decanters and super-pressure system [SP]) during the 2017 growing season. Physicochemical and microbiological characteristics of these effluents were examined. OMW phenolic extracts were prepared and their antioxidant activities were evaluated. Moreover, the improvement of virgin olive oil stability by the addition of OMW phenolic extracts (at 100, 300 and 500 ppm) was investigated. Results from the present work confirmed the pollutant load of these effluents characterized by an acidic pH, strong organic content, and considerable amounts of mineral matter. Concerning the microbiological counts, yeasts and molds were the most abundant. Among extraction system, C2 displayed OMW with the high levels for all measured parameters, except for total phenols that were more abundant in C3. Findings from DPPH and FRAP tests showed a good antiradical potential and an important reducing power capacity and were both concentration-dependent. For the two tests, antioxidant activity of OMW phenolic extracts from C2 was the most important. The addition of phenolic extracts to virgin olive oil, stocked at 60°C, resulted in lower values of PV, K232 and K270 compared to the control, indicating a good protective effect against oil oxidation. The great antioxidant effect of OMW extracts was at 500 ppm, similar to that of ascorbic acid
