Volatile organic compounds (VOCs) are harmful to the environment and human health, which can be effectively removed by catalytic oxidation processes. In this work, OMS-2 was modified by adding different amounts of K via a one-pot hydrothermal process to obtain samples with unique hierarchical porous structures, named OMS-2-xA (x = 1, 1.5, 2.5 and 3.5), and these samples were applied for catalytic oxidation of toluene and benzene. Various characterizations including XRD, XRF, N₂-adsorption–desorption, SEM and TEM were carried out to explore the textual properties and morphology of samples, confirming that the OMS-2-xA samples showed different K contents and morphologies with hierarchical structures of pure cryptomelane crystalline phase. The measurements of XPS, H₂-TPR and O₂-TPD were made to investigate the influence of different contents of K in the OMS-2-xA samples on their surface Mn species, lattice oxygen species and oxidation ability. Additionally, in situ DRIFTS tests were used to explore the difference in the activity and mobility of oxygen species between OMS-2-2.5A and traditional OMS-2. Among these samples, OMS-2-2.5A with appropriate K amounts presented the optimal catalytic oxidation performance on toluene (T₉₀ of 207 °C) and benzene (T₉₀ of 233 °C), which was better than that of the conventional OMS-2. A much higher Mn⁴⁺ content and better oxygen mobility due to the formation of more surface K–O–Mn bonds result in its excellent catalytic oxidation activity for toluene and benzene.