© 2019 Elsevier Ltd Hydrothermal carbonization (HTC) was used to treat olive oil mill wastewater (OOMW) by using water in the absence of any additional reagent. The reaction time and the water-sludge (W/S) ratio were varied, over a broad set of values, to study their effect on the hydrochar products. As reaction time and water-sludge ratio increased, the hydrochar yield decreased from 36% to 7%. In addition, HTC upgraded both carbon and energy contents to very high values, 76% and 36 MJ/kg respectively. Hence, the hydrochars products are good candidates for energy generation. Moreover, the hydrochars products were amorphous and had a hydrophobic oleophilic structure. Their quality and energy yield increased with the decrease in water-sludge ratio while reaction time slightly affected the hydrochar properties. Operating the process at the lowest water-sludge ratio and short reaction time leads to hydrochars with the finest characteristics but at the expense of higher cost due to evaporation rate.