Abstract <p>This study reports the synthesis of Li₂O–K₂O/CaO nanocatalyst via direct impregnation of discarded chicken eggshells with a mixture of LiNO₃ and KNO₃ at various mass ratios, followed by thermal calcination at different temperatures for varying durations at a heating rate of 10°C/min. The typical nanocatalyst (15.0 wt % Li₂O–K₂O/CaO) was analyzed by XRD, FESEM, TEM, EDX, and BET surface area. This nanocatalyst was utilized in the cosolvent-assisted transesterification of waste cooking oil with methanol. The optimization of conditions influencing biodiesel yield was achieved using a Box-Behnken design and response surface methodology (BBD-RSM). The optimized parameters included the catalyst amount, the methanol: WCO molar ratio, the cosolvent transesterification temperature, and the duration. Based on the optimization results, the highest BD yield (96.0%) was attained at 60°C with a 6:1 methanol: WCO ratio for 90 min using 3.0 wt % of the Li₂O–K₂O/CaO nanocatalyst. ¹H NMR measurements indicated a conversion of 98.85%. Furthermore, the nanocatalyst demonstrated a reasonable biodiesel output in the 4th reuse cycle. The synthesis of a catalyst from eggshells offers numerous benefits, making it a sustainable catalyst for biodiesel production<i>.</i></p>