To monitor ozone (O3) concentration in the environment, it is urgent to develop highly sensitive O3 gas sensors with low limit of detection (LOD) and relatively low working temperature. Herein, a series of nanosheet-assembled In2O3 with different chemisorbed oxygen components were prepared successfully. The hierarchical In2O3 sensing materials possessed the loose and porous structure. The O3 sensing results indicated that the adjustment on the amount of chemisorbed oxygen finely enhanced the response of the sensor. As an efficient O3 detector, the sensor exhibited an outstanding response of 16.7 to low concentration of O3 (100 ppb). And the LOD was measured as low as 30 ppb at a relatively low operating temperature (80 °C). In addition, it is observed that the O3 sensing behaviors were interfered by oxygen (O2) at the background. The response of the sensor was lower at the background of pure O2 than that at the background of air. Competitive adsorption between O3 and O2 molecules might exist and was investigated. Taken together, this work expands a new perspective to design highly superior O3 gas sensors.