A process for upgrading a heavy oil includes passing heavy oil and disulfide oil to a thermal cracking system that includes a thermal cracking unit and a cracker effluent separation system downstream of the thermal cracking unit and thermally cracking at least a portion of the heavy oil in the presence of the disulfide oil in the thermal cracking unit to produce solid coke and a cracking effluent comprising reaction products. The reaction products include one or more liquid reaction products, one or more gaseous reaction products, or both. The presence of the disulfide oil in the thermal cracking unit promotes conversion of hydrocarbons from the heavy oil to the liquid reaction products, the gaseous reaction products, or both relative to the production of the solid coke.