Three reactors were designed and used for the microwave remediation of oil-polluted wastes in a high-power microwave multimode cavity; a pressure model was also built based on reactor shapes and sizes. It was found that the reactor shape and size can extremely affect the electric field strength and distribution and, therefore, the efficiency of oil removal from the wastes. The reactor size would affect the pressure produced during treatment and thus affect the boiling point of the oil. The sweet gas distribution is also a factor for oil removal, although the flow rate has little effect on the oil removal when a uniform distribution is applied with a specific reactor. It was verified that multimode cavities are best-suited for big volume samples, in the case of uniform heating.