Four kinds of cryptomelane-type octahedral molecular sieve (OMS)-2-X (the X represents the molar ratio of KMnO4/MnAc2) were prepared as catalytic materials for ozone decomposition through a one-step hydrothermal reaction of KMnO4 and MnAc2, by changing their molar ratios. These samples were characterized by N2 adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), temperature programmed reduction by H2 (H2-TPR) and X-ray photoelectron spectroscopy (XPS). Among them, the OMS-2-0.7 sample showed the best O3 conversion of 92% under high relative humidity (RH) of 90% and gas hourly space velocity of 585,000 h−1. This was accordingly thought as a possible way for purifying ozone-containing waste gases under high RH atmospheres. The efficiency of ozone decomposition of the prepared OMS-2-X sample was found to be related to specific surface area, particle size, surface oxygen vacancies, and Mn3+ cation amounts. The one-step hydrothermal synthesis was shown to be a simple method to prepare the considerably active OMS-2 solids for ozone decomposition.