Oxygen loss is widely recognized as a major factor in the structural degradation of Ni-rich layered cathode materials, while a study on the detailed oxygen evolution process and the role of inevitable Li/Ni disordering defects is still lacking. Using ab initio calculations with van der Waals correction, we performed an in-depth study on the oxygen evolution in layered LiNiO2 and considered the role of Li/Ni disordering. We find that excess Ni in Li slab act as a pillar role for the structural framework and increases bond strength of adjacent oxygen ions, both of which are beneficial to the thermal and kinetic stability of lattice oxygen in LiNiO2, thus inhibiting the oxygen evolution. For Li/Ni exchange and Excess Li defects, the Li ion in Ni slab preferred to return to the vacancy in Li slab with delithiation in both surface and bulk LiNiO2. This situation induces the weaken binding and fast migration of lattice oxygen, even Ni slab collapse and oxygen release in the surface. Furthermore, the ab-initio molecular dynamics simulations for LiNiO2 surface reveal three stages of oxygen loss in the pristine state: (i) the formation of pre-O-O dimer; (ii) the pre-O-O dimer oxidizing to O-O dimer; (iii) O2 release with oxygen oxidation and Ni migration. The Li/Ni exchange and Excess Li defects accelerate this process because of Li ion in Ni slab returning to Li slab, while the Excess Ni defects will inhibit this process. We hope this work will improve the understanding of oxygen evolution in layered LiNiO2 and provide theoretical guidance for the design of Ni-rich layered cathode materials with improved stability.