The organic wastes including olive oil mill residues are an inseparable part of food manufacturing processes while implying multi-faceted damages to the environment. A good quantity of research has been conducted to examine the biogas enhancement level in the anaerobic process of olive residues.  Seeking the optimum pretreatment method and the co-digesting substrates, the current study has conducted aggregative research on 155 experiments out of 22 studies. The conducted meta-analysis recognized the chemical type of pretreatments as the most effective treating procedures, according to which, application of the combined alkaline and lime, followed by trace metal cobalt supplementations are recognized as the most effective methods.  Furthermore, the study found intriguing results on the optimum type of olive main substrate, inoculum, digester type and effective volume as well as the superior country and year, in the anaerobic digestion of the olive mill residues.