Olive mill wastewater, hereafter noted as OMWW was tested for its composition in phenolic compounds according to geographical areas of olive tree, i.e. the plain and the mountainous areas of Tadla-Azilal region (central Morocco). Biophenols extraction with ethyl acetate was efficient and the phenolic extract from the mountainous areas had the highest concentration of total phenols' content. Fourier-Transform-Middle Infrared (FT-MIR) spectroscopy of the extracts revealed vibration bands corresponding to acid, alcohol and ketone functions. Additionally, HPLC-ESI-MS analyses showed that phenolic alcohols, phenolic acids, flavonoids, secoiridoids and derivatives and lignans represent the most abundant phenolic compounds. Nüzhenide, naringenin and long chain polymeric substances were also detected. Mountainous areas also presented the most effective DPPH scavenging potential compared to plain areas; IC50 values were 11.7 ± 5.6 µg/ml and 30.7 ± 4.4 µg/ml, respectively. OMWW was confirmed as a rich source of natural phenolic antioxidant agents.