Heavy metals like Cu(II) and oxytetracycline (OTC) frequently coexist in livestock manure and wastewater due to their widespread use as feed additives, forming Cu(II)-OTC complexes that influence OTC transport and transformation in groundwater. This study investigates Cu(II)-mediated OTC transformation on Fe-bearing smectite under dark, anoxic conditions. Results show that OTC adsorption on Fe-bearing smectite undergoes further transformation. Cu(II)-OTC complexes significantly enhance OTC adsorption, accounting for 23.6%, 99.4%, and 92.9% of total OTC at pH 3, 5, and 8.5, respectively, but inhibit OTC transformation. FTIR, XPS, and UPLC-QTOF analyses reveal redox reactions between Cu(II)-OTC complexes and Fe-bearing smectite, reducing Fe(III) to Fe(II). Transformation primarily involves the tricarbonylamide group at pH 3 and 5, extending to the dimethylamino group at pH 8.5. A kinetic model of OTC transformation identifies two key parameters: the transformation rate (k') and the reactive site quantity (S