<p>Oily sludge is a hazardous by-product of petroleum refining, characterized by high concentrations of recalcitrant hydrocarbons and organic matter that limit the effectiveness of biological treatment alone. In this study, a homogeneous Fenton advanced oxidation process was optimized as a pretreatment step to enhance subsequent aerobic composting of refinery oily sludge. The effects of pH (3–5), hydrogen peroxide dosage (8–12 mL g⁻¹ oily sludge), and Fe²⁺/H₂O₂ molar ratio (0.10–0.16) on chemical oxygen demand (COD) removal were investigated using a Box–Behnken response surface methodology. Under optimized conditions (pH = 5.0, H₂O₂ = 8.5 mL g⁻¹ oily sludge, Fe²⁺/H₂O₂ = 0.10), the model predicted 96.5% COD removal, while experimental validation confirmed approximately 94%. The Fenton pretreatment also achieved 98.2% total petroleum hydrocarbons (TPH) removal, significantly reducing pollutant load and improving suitability for subsequent biological treatment. The pretreated sludge was composted aerobically for 75 days using different microbial inoculum. Fruit and vegetable compost showed the highest additional hydrocarbon degradation (20.3%, relative to residual hydrocarbons after Fenton pretreatment), while improving compost maturity (germination index (GI) &gt; 80%). Overall, the integrated Fenton–composting system achieved 98.6% cumulative TPH removal relative to untreated sludge. These findings demonstrate an effective strategy for detoxification, stabilization, and partial valorization of refinery oily sludge within a circular-economy framework. To the best of our knowledge, this study is among the first to systematically integrate optimized Fenton oxidation with aerobic composting while simultaneously evaluating oxidation efficiency and compost quality.</p> Graphical abstract <p>Graphical abstract summarizing the integrated Fenton–composting process and its circular‑economy implications.</p> <p></p>