The catalysts for low-temperature selective catalytic reduction of NO with NH (NH-SCR) are highly desired due to the large demand in industrial furnaces. The characteristic of low-temperature requires the catalyst with rich active sites especially the redox sites. Herein, the authors obtain oxygen defect-rich β-MnO from a crystal phase transformation process during air calcination, by which the as-prepared γ-MnO nanosheet and nanorod can be conformally transformed into the corresponding β-MnO. Simultaneously, this transformation accompanies oxygen defects modulation resulted from lattice rearrangement. The most active β-MnO nanosheet with plentiful oxygen defects shows a high efficiency of > 90% NO conversion in an extremely wide operation window of ≈120–350 °C. The detailed characterizations and density functional theory (DFT) calculations reveal that the introduction of oxygen defects enhances the adsorption properties for reactants and decreases the energy barriers of *NH formation more than 0.3 eV (≈0.32–0.37 eV), which contributes to a high efficiency of low-temperature SCR activity. The authors finding provides a feasible approach to achieve the oxygen defect engineering and gains insight into manganese-based catalysts for low-temperature NO removal or pre-oxidation.