The structure and reactivity of manganese oxides (MnOx) are sensitive to the solution environment. To explore the effects of dissolved oxygen (DO) on the formation of MnOx in manganese removal filters, two identical filters were initialized by dosing potassium permanganate and manganese chloride with different influent DO concentrations. During the start-up process, the structural characteristics of the MnOx filter media in two filters were probed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Brunauer-Emmett-Teller (BET) specific surface area, and X-ray photoelectron spectroscopy (XPS). Results of XRD analyses revealed that DO had great influence on the crystal structure of the samples. The crystallization of the MnOx samples in filter D1 (DO= 6.5 +/- 0.5 mg/L) gradually increased with running time, while the MnOx samples in filter D2 (DO= 1.5 +/- 0.5 mg/L) remained the same. The morphologies of the MnOx changed from aggregated particles to a wrinkled shape in both filters. Results of compositional analyses showed that the high DO condition was beneficial for the insertion of Ca, as well as the ion exchange of Ca and K, in the interlayer region of the MnOx. The MnOx that formed at high DO conditions possessed greater manganese removal activity than that formed at low DO conditions; this was related with the abundant lattice oxygen. Vacancy defects played an important role in the removal of manganese at low DO conditions. Through this study it is suggested that aqueous DO is a critical environmental factor controlling MnOx structure and reactivity in the environment. This study provides significant insights into future applications of the manganese removal techniques.