Olive mill wastewater sludge (OMWS) and waste tires (WTs), abundant wastes in Tunisia, were used as feedstock in a slow co-pyrolysis pilot reactor to produce upgraded pyrolytic oil as an alternative fuel. Despite the improvement of some properties of the pyrolytic oil when waste tires were added in the feed blend, a negative synergy was observed in the yield of the oil compared with that of char. The characterization of oil samples showed synergetic interaction between OMWS and WTs during co-pyrolysis which led to a partial deoxygenation and resulted in reduction of viscosity and increase in the calorific value of the co-pyrolytic oils. However, the co-pyrolytic oil properties did not meet the requirements of commercial diesel and will need further improvement by effective standardization to meet marketable specifications. Compared with catalytic fast pyrolysis (CFP) followed by hydrodeoxygenation (HDO), OMWS/WTs slow co-pyrolysis showed some limitations but it can be considered as a simple, clean and cheap process upgrading technique for bio-oil production (∼40% lower in fixed capital investment and ∼30% lower in fuel selling price).