A solid heterogeneous catalyst was derived from oil shale ash by impregnation of the ash with KNO3 followed by calcination for 4 h. Different preparation conditions were studied (KNO3 concentrations: 0.05 and 0.1 M, and calcination temperatures: 500 and 700 °C). After calcination, the dependence of waste cooking oil to biodiesel conversion on the reaction variables such as the catalyst loading, the methanol to oil molar ratio, and reaction time was investigated. The catalyst characterization was conducted using FT-IR, XRD, BET, TEM, and SEM. Further, the typical unsaturated fatty acids present in common vegetable oils, the oil-to-biodiesel conversion, and the chemical composition of the produced biodiesel were quantified by (¹H NMR). Among the various catalysts prepared, the ash impregnated with 0.1 M KNO3/and calcined at 700 °C (Ash 0.1/700 catalyst) provided the maximum oil to biodiesel conversion of about 100% at 65 °C reaction temperature, methanol to oil molar ratio of 45:1, and 2 h reaction time.