Introducing a surface compressive stress layer is an effective way to enhance the strength of brittle ceramics, yet achieving such prestressing intrinsically in monolithic oxide ceramics remains challenging. Here, we report a novel method called oxygen-vacancy compensation prestressing (OVCP) to generate in situ surface prestressing in zirconia-toughened alumina (ZTA) ceramics. Oxygen vacancy-rich ZTA was first produced by vacuum hot pressing, followed by air annealing to induce surface reoxygenation and form an oxygen-charged layer (OCL). The optimized treatment increased the flexural strength to (1679±78) MPa, representing a 31% improvement over the unannealed state. Oxygen-vacancy compensation during annealing induces lattice expansion in the near-surface region. Constrained by the less-oxidized interior, this lattice expansion is converted into a residual compressive stress field that suppresses bending-induced failure. A simplified bilayer model quantitatively supports the experimentally observed strengthening behavior. These findings establish oxygen vacancy-regulated lattice expansion as an effective mechanism for intrinsic surface prestressing and provide a simple, interface-free route for strengthening oxide ceramics.