Abstract Nanostructured calcium oxides supported onto biochar obtained by pyrolysis of avocado seeds were prepared, characterized and successfully used as catalysts to produce biodiesel from waste oils. The effect of increasing calcium load (5, 10 and 20 wt%) was investigated. Elemental analysis, FTIR, XRD, SEM, BET, acid and basic sites were used to characterize the resulting carbon-based calcium oxides. Supported systems efficiently promoted the transesterification of oil with methanol, but differently from calcium oxide, they were easily recoverable and reusable for three cycles without any loss of activity. Kinetic data were better fitted by a pseudo-second order model with an activation energy of 39.9 kJ mol−1. Thermodynamic parameters of activation energy were also determined for the transesterification reaction ( Δ ‡ G: 98.68-106.08 kJ mol−1, Δ ‡ H: 37.05 kJ mol−1 and Δ ‡ S: 0.185 kJ mol−1 K). Finally, reaction conditions were optimized using the desirability function applied on the response surface methodology analysis of a Box–Behnken factorial design of experiments. By carrying out the reaction at 99.5 °C for 5 h with 7.3 wt% of catalyst and a molar ratio of methanol to oil of 15.6, a FAME content over 96% was achieved. Even starting from waste cooking oil, final biodiesel was conform to the main EN14214 specifications.