The expression of laccase and manganese peroxidase genes of a selected strain of Pleurotus ostreatus were studied in olive oil mill wastewater (OMW). The fungal strain decolourized 50% OMW in a linear way for 21 days and, at the same time, degraded the phenol compounds by 85%. Transcripts of laccase genes poxa1b, pox2, poxa3, and sspoxa3a, sspoxa3b coding for the small subunits of POXA3, were estimated by qRT-PCR, at different time intervals, together with beta-tubulin gene used as internal control, from fungal cultures grown in a chemically-defined complete medium (CM), a supplemented CM with the addition of Cu+2 and Mn+2 (CM-plus) and 50% OMW in distilled water. The most abundant transcripts in both OMW and CM-plus were those of the poxa3, whereas pox2 transcripts were induced only in OMW and those of poxa1b at a strict time-window (14 days) in both OMW and CM-plus. Interestingly enough, the transcripts of genes sspoxa3a and sspoxa3b were up-regulated between 14-21 days, at a time at which the large subunit of the enzyme coded by poxa3 was down-regulated. The manganese peroxidase gene mnp2 exhibited a strong and specific transcriptional induction in OMW after 12 and 14 days, followed by a drastic drop after 18 days and a complete cease of expression at day 21, whereas mnp3 transcripts were at maximum level in OMW at day 10 but where thereafter reduced.