Commercially valuable oil is recovered from refinery-produced, Resource Conservation and Recovery Act classified, hazardous waste oil sludges by a series of volatilizing and condensing steps (which may vary depending on the oil, water, and solids content of the sludge) resulting in the production of nonhazardous water and solid waste byproducts. A pumpable, low viscosity, high oil- and/or water-content sludge is first centrifuged to separate free oil and water. If the waste sludges are acidic or if nickel is present in the sludges in sufficient quantities so as to result in unacceptable levels in the TCLP leach filtrate, a base (such as calcium oxide) is admixed with the thick waste oil sludge, centrifuge solids, or other sludges low in oil and/or water content to render the sludges basic. The basic, admixed sludges are heated to volatilize the contained water and oil. Dry, friable, deoiled solids are recycled with the oily sludge feed to prevent material agglomeration and heat exchanger fouling. Oil and water vapor from the high-temperature volatilization apparatus is recycled as an indirect heat source to the low-temperature volatilization apparatus. The volatilized oil and water are condensed and combined with the oil and water centrate, and the oil and water are essentially separated as by gravity. The oil is separated from any entrained water and solids. The nearly oil free, nonhazardous waste water and nonhazardous, deoiled solids can be disposed of in a conventional manner, and the commercially valuable recovered oil is suitable for further refinery processing.