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An ultrasensitive label-free electrochemical immunosensor of okadaic acid (OA) was studied based on electrodimerization of p-aminobenzoic acid (p-ABA)-modified gold three-dimensional nanoelectrode ensembles (3DNEEs) which were prepared by electroless gold plating and etching using polycarbonate as template. Staphylococcus protein A (SPA) was introduced to immobilize okadaic acid antibody (anti-OA). The morphology and composition of 3DNEEs were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDX). The polydimer (p-aminobenzoic acid) (pPABA) film was measured by Fourier transform infrared spectrometer (FTIR). The concentration of OA was measured by EIS and SWV. The detection limit of EIS is lower than that of SWV (6.47 × 10−3 ng mL−1 vs. 9.15 × 10−3 ng mL−1), but the linear OA detection range of the latter is wider (0.01–1.0 ng mL−1 vs. 0.01–0.8 ng mL−1). The average recovery of OA in oyster samples is 99.81%. The OA recovery test in oyster samples demonstrated potential application value of the electrochemical immunosensor.