A novel method to remove the carbon residue precursor from waste plastic pyrolysis oil has been developed to improve subsequent pyrolysis oil refining efficiency by reducing fouling. The carbon residue content of the pyrolysis oil that cannot be filtered is reduced by precipitating the carbon residue precursor from the pyrolysis oil under mild conditions. By emulsifying an aqueous solution of oxidant and the pyrolysis oil, the carbon residue precursor was oxidized at the oil-water interface without oxidizing the pyrolysis oil. Enhancing intermolecular interaction by hydrophilic functional groups formed by oxidation induces the precipitation of carbon residue precursors. The precursor removal was determined by the type and reaction time of oxidants. FeCl3 and H2O2 are efficient oxidants, and recycling those oxidant solutions can also remove the carbon residue precursor. The number of recycles with the precursor removal effect was determined by the amount of oxidant remaining in the aqueous solution. Also, a short 15-minute FeCl3 reaction can eliminate the precursor. Reducing the reaction time is expected to increase process efficiency, as unnecessary oxidation and energy consumption are decreased. Our research suggests the pretreatment of the pyrolysis oil to reduce the carbon residue content, thereby reducing the fouling during the subsequent pyrolysis oil refining.