The effects of adding solid additive Oyster Shells (OS) and liquid additive Olive Mill Waste Water (OMWW) on product yields and properties were investigated and presented during the slow pyrolysis of Olive Pomace (OP) in a lab-scale batch reactor. OP impregnation with OMWW has improved the bio-oil production, with a maximum yield of 36.3 wt %. The maximum bio-char yield, however, was obtained with the OS addition (10 wt%) and was 44.1 wt%. The Micro-GC technique was used to determine the composition of the pyrolytic gases. Following the Micro-GC results, CO2, CO, CH4 and H2 are the main components present in pyrolytic gas mixture. The highest yields of energetic vectors CH4 (22.7%) and H2 (18.7%) were recorded in the absence of any additives. In terms of the environment, adding 10 wt% of OS was beneficial as it cuts down the emission of greenhouse gases (CO2 and CO) to over 50% through absorption and shift water reactions. The Gas Chromatography/Mass Spectrometry GC/MS results showed that the pyrolytic oils were composed of phenolic compounds (33.07–58.23 PA%), acids (2.92–39.31 PA%), alcohols (3.03–10.04 PA%), aldehydes (2.81–8.29 PA%), ketones (0.97–11.46 PA%), esters (6.06–14.83 PA%). The OP impregnation with OMWW improves pyrolytic liquid quality by minimizing its corrosiveness and its instability. In fact, the lowest acids (2.9 PA %) and aldehyde (2.8 PA %) contents were obtained by OP impregnation. On the other hand, OS addition leads to an increase in the high value-added products, such as guaiacol (from 10.96 to 14.3 PA %) and syringol (from 10.48 to 13.6 PA %). Both OS addition and OP impregnation were proven to improve the bio-char quality by increasing its carbon content respectively by 14.9% and 18.4% and decreasing its oxygen content by 14.5 and 17.5%, respectively. Therefore, higher calorific values were recorded 22.6 and 23.7 MJ/kg, respectively. These calorific values suggest the suitable use of bio-chars as solid fuels. © 2023 Elsevier Ltd