Spin–orbit torque (SOT) is recognized as an effective way to manipulate magnetization in spintronic devices. For the low-power consumption and high-endurance requirements of future computer architectures, reducing the critical SOT switching current density and improving SOT efficiency are crucial, especially in the perpendicularly magnetized structures. Here, we have conducted a comprehensive study on improving the SOT efficiency of the Ta/CoFeB structure with a perpendicular magnetic anisotropy by inserting an oxide insulating layer Fe2O3 as the bottom layer. We found that only a 1–5 nm thickness of Fe2O3 significantly reduces the SOT critical switching current by 70% and enhances the spin Hall angle of Ta. The spin Hall angle increases from 0.078 for pure Ta/CoFeB to 0.13 for Fe2O3/Ta/CoFeB, and both types of spin–orbit torques, damping-like and field-like torques, are significantly enhanced. It is suggested that the atomic diffusion of O from the Fe2O3 underlayer leads to the partial oxidization of the Ta layer as well as the Ta/CoFeB interfaces, accounting for the observed enhanced SOT efficiency. Our results provide a reliable method to improve the SOT performance in perpendicularly magnetized structures by inserting the oxide underlayer using magnetron sputtering, in favor of its potential real-world application in spintronic devices.