Hafnium oxide (HfO₂)-based ferroelectric memory devices have emerged as promising CMOS-compatible nonvolatile memories, yet ferroelectric synaptic devices still suffer from polarization-state read-disturb issues and abrupt polarization variations in neuromorphic computing applications. Here, we demonstrate a ferroelectric synaptic device with highly linear and continuously tunable multilevel polarization states as well as improved resistance to read-disturb issues through electrically driven oxygen-vacancy migration in Hf_0.5Zr_0.5O₂ (HZO) devices. Using an oxygen-active indium tin oxide (ITO) electrode, electric-field cycling induces interfacial oxygen-vacancy redistribution, leading to vacancy compensation and interface stabilization. Spectroscopic and nanoscale analyses reveal field-driven redox processes, including Ti³⁺ to Ti⁴⁺ transformation and three-dimensional oxygen-vacancy migration across the interface. This dynamic defect regulation enhances remanent polarization (P_r), increases breakdown strength, and enables robust endurance. Importantly, controlled vacancy migration allows continuous P_r modulation, supporting multilevel analog synaptic behavior with high uniformity and reversibility. Implemented in a 1T1C architecture, the device exhibits a large and linear memory window with excellent retention. These results establish electrically driven oxygen vacancy engineering as an effective post-fabrication strategy for high-reliability ferroelectric synaptic devices.