•Review study for the main characteristics of olive mill solid waste to energy practices undertaken the last two decades.•Theoretical background of the main waste-to energy conversion pathways of solid olive mill wastes are presented.•The state of the art of pre-treatment techniques for upgrading solid fuels are described.•OMSW related research must focus on the optimization of the proposed conversion technologies. In recent years, the utilization of olive industry by-products for energy purposes has gained significant research interest and many studies have been conducted focused on the exploitation of olive mill solid waste (OMSW) derived from the discontinuous or continuous processing of olive fruits. In this review study, the primary characteristics of OMSW and the techniques used to define their thermal performance are described. The theoretical background of the main waste-to-energy conversion pathways of solid olive mill wastes, as well as the basic pre-treatment techniques for upgrading solid fuels, are presented. The study aims to present the main findings and major conclusions of previously published works undertaken in the last two decades focused on the characterization of olive mill solid wastes and the utilization of different types of solid olive mill residues for energy purposes. The study also aims to highlight the research challenges in this field.