In this study, biological methods (biostimulation and bioaugmentation) were used to treat oil tank bottom sludge contaminated soils to total petroleum hydrocarbon (TPH) levels suitable for landfill disposal. The sludge’s hydrocarbon-degrading microbial capacities were initially compared to those from other contaminated environments using culture-based methods. Results indicated that a fungus, Scedosporium dominated the sludge microbial community. Its application in a nutrient formulation resulted in greater reduction in oil tank bottom sludge viscosity (44 %) and residual soil hydrocarbon compared to hydrocarbonoclastic microorganisms from other sources (26.7 % reduction in viscosity). Subsequent field-based experiments showed greater TPH reduction (54 %) in fungal-nutrient-treated sludge–waste soils than in naturally attenuated controls (22 %) over 49 days. 16S ribosomal ribonucleic acid and internal transcribed spacer region-based polymerase chain reactions and denaturing gradient gel electrophoresis analyses showed minimal effects on the microbial communities during this time. TPH reduction to landfill disposal levels occurred at a slower rate after this, falling below the 10,000 mg kg−1 legislated TPH disposal threshold earlier in amended samples (91.2 %; 9,500 mg kg−1) compared to the control (82 %; 17,000 mg kg−1) in 182 days. The results show that the intrinsic hydrocarbon-degrading microbial capacities in sludge are better suited for sludge degradation than those from other sources. The substantial TPH reduction observed in control samples demonstrates the beneficial effects of natural attenuation with waste soils for oil tank sludge treatment. Microbial capacities in sludge and treated waste soils can therefore be successfully employed for treating oil tank bottom sludge.