A method and apparatus for recovering oil from oil-containing sorbents, such as drill cuttings obtained from drilling with an oil-based mud. The method includes peptizing the substrate with an acid reagent and direct thermal desorption with combustion effluent gases at high temperature under turbulent mixing conditions. Another method disclosed includes upgrading the oil in the substrate to improve one or more of the properties of the recovered oil relative to the oil in the substrate, such as, lower aromatics content, lower sulfur content, lower functional group content, higher saturates, higher viscosity, higher viscosity index, and any combination thereof. The apparatus provides for efficient recovery of oil from the substrate with a short residence time, high throughput, low residual oil content in the treated solids and/or high percentage of oil recovery. The apparatus may be transported to a remote location for on-site treatment of drill cuttings or other oil-containing solids.