Abstract The ability of seven distinct species of white-rot basidiomycetes (WRB) to produce lignin-modifying enzymes (LME) and reduce high phenolic content was assessed during submerged fermentation of oil mill by-products. Cerrena unicolor strains appeared to be the outstanding producers of laccase (56.3–78.5 U mL−1) and MnP (1.03–2.76 U mL−1) in the fermentation of olive pomace (OP), and laccase (26.9–93.8 U mL) in the fermentation of olive tree sawdust. Diluted 50% olive mill wastewater (OMW) can be used as a nutrient medium for laccase and MnP production but its supplementation with additional carbon and nitrogen sources and Cu2+ and Mn2= promoted the 3-, 28- and 18-fold increase of C. unicolor biomass and volumetric laccase and MnP activities, respectively, as well as 3- and 17-fold increase of Pleurotus ostreatus biomass and laccase activity, respectively. The tested fungi successfully reduced the phenolics content in the submerged fermentation of OP (72.0–81.3%) and OMW (62.4–83.5%). The laccase role in the OP phenolics reduction was proved through the treatment of 10% (wet weight) OP with commercial and lab-generated laccases preparations and reduction of total phenols by 66–70% during 24 h incubation. This work indicated that the olive mill by-products fermentation by individual substrate-specific WRB is an economically feasible and promising alternative for the simultaneous production of LME, cellulase, xylanase and detoxified materials suitable for further exploitation in biotechnological and agronomic applications. Highlights • Submerged fermentation of olive mill by-products was performed. • Selected white-rot fungi successfully produced laccase and manganese peroxidase. • Cerrena unicolor is the outstanding producer of laccase (78.5 U mL−1) and MnP (2.76 U mL−1). • Total phenolics reduction of 62.4–83.5% in olive pomace and wastewater was achieved. • The laccase role in the total phenolics reduction was proved. [ABSTRACT FROM AUTHOR]