Abstract: The production and disposal of large amounts of oil sludge are considered a most critical environmental issue in the petroleum industry. The possible conversion of oil sludge produced in a fuel oil storage tank to porous carbons by simple pyrolysis or KOH activation was investigated and the feasibility of their use to treat drinking water and to adsorb were evaluated. The oil sludge contains 80% of carbon and consists of mainly aliphatic compounds. The porous carbon obtained by KOH activation has a BET surface area, total pore volume and micropore surface area of 328.0 m²·g^-1, 0.21 cm³·g^-1 and 289.10 m²·g^-1, respectively, while that produced by pyrolysis has a much lower surface area of 3.6 m²·g^-1. Although Cd, Cu, Zn, Mn, and Fe are enriched in the porous carbons compared with the oil sludge, their leaching in distilled water is low and below the allowed standard limits except for Cr which is close to the limit. The Cd adsorption removal rates for the KOH activated porous carbon and the pyrolysed one are 97.36 and 77.74%, respectively. The former is comparable to three commercial activated carbons under the same conditions. The recovery of the oil sludge through KOH activation to prepare porous adsorbents for waste water treatment can be suggested as an alternative to the conventional disposal methods.