The challenges of voltage decay and irreversible oxygen release for lithium-rich layered oxide cathode materials have hindered their commercial application despite their high energy density and low cost. Herein, a facile post-annealing strategy is developed to pre-introduce oxygen vacancies (OVs) into Li1.2Mn0.457Ni0.229Co0.114O2 cathode materials. The induced OVs modify the local Mn coordination environments, enhance structural stability, and suppress oxygen release. The modified cathode exhibits a discharge capacity of 224.1 mAh g−1 at 0.1 C after 100 cycles with 97.7 % capacity retention. Even at 2 C, excellent capacity retention of 93.3 % after 300 cycles can be achieved. In situ and ex situ X-ray diffraction are used to elucidate the reaction mechanisms and crystal structure during cycling tests. Ex situ X-ray photoelectron spectroscopy confirmed the suppressed oxygen release, enhanced oxygen vacancies and reduced cathode-electrolyte interfacial layer after cycling for the post-annealed cathode. Our results show that the presence of oxygen vacancies through thermal expansion diminishes the phase transitions in cathode materials during the heating process. These findings contribute to developing next-generation Li-ion batteries (LIBs) by oxygen vacancy engineering for new cathode materials with improved electrochemical performances.