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The development of waste-derived adsorbent is of great significance to realize the idea of "treating waste with waste". Herein, a novel porous activated carbon was synthesized from oil sludge waste (OSAC) in a facile way. The resultant OSAC possesses favorable properties such as high specific surface area (324.98 m2 g−1) and abundant surface oxygen-containing functional group, which contribute to outstanding adsorption capabilities for Methylene Blue (MB, 316.02 mg g−1) and Direct Blue 6 (DB, 124.24 mg g−1) dyes. The adsorption capacity of OSAC is closely related to the initial solution pH since the surface charge state of the adsorbent is affected by pH values. With the increasing of pH values, the adsorption capacity of MB raised while that of DB decreased. The adsorption isotherms, kinetics, thermodynamics analyses have been investigated. Although the Langmuir model and the pseudo-second order model fitted well to the experimental data for both dyes and the thermodynamic parameters revealed their adsorption processes were all endothermic and spontaneous, the adsorption mechanism of the two dyes is not exactly the same. In addition to the difference of physical adsorption caused by dyes molecular size, the difference in chemical adsorption effects due to the different charge properties of the two dyes was also an important aspect of the adsorption mechanisms. Moreover, the reutilization analysis shown a presentable recyclability of the OSAC. It is demonstrated that the strategy developed here would provide cost-effective production of adsorbents for treatment of dye wastewater and offer a promising avenue of fabrication of high-value-added products from hazardous industrial waste. [ABSTRACT FROM AUTHOR]