Siderite oxidation can be achieved with dissolved oxygen (DO), which would control the oxidation and the fate of As(III), but the interaction mechanism between siderite and As(III) with the presence of DO needs more investigation. Herein, As(III) adsorption and oxidation on siderite with DO were investigated systematically through batch kinetics in combination with Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy depth profiling, and quenching experiments. Results showed that As(III) adsorption increased with increasing DO concentrations from 0 to 4.6 mg L⁻¹, but stabilized at a DO concentration of 9.0 mg L⁻¹. Siderite was oxidized to form Fe (hydr)oxides with DO, which promoted As(III) adsorption. However, with excess DO, siderite was intensively covered by newly formed Fe (hydr)oxides, which limited further oxidation of siderite and As adsorption. During the reaction of structural Fe(II) with DO, ·OH was generated by the Fenton-like reaction, which oxidized the adsorbed As(III). As(III) adsorption was quicker than its oxidation because of the higher As(III) proportion on the reacted siderite. These findings provide new insights into the interaction mechanism between As(III) and structural Fe(II) in the presence of DO.