<p>This record presents a first-principles oxygen K-edge X-ray absorption spectroscopy (XAS) dataset for the Ni-rich layered cathode material NMC811 (Li₁₋ₓNi₀.₈Mn₀.₁Co₀.₁O₂) across three states of charge. Nine NMC811 supercells were considered: three low-energy cation/vacancy configurations at each of three lithiation levels (pristine x = 0.0, half-delithiated x = 0.5, and fully delithiated x = 1.0), each supercell containing 60 transition-metal sites (48 Ni, 6 Mn, 6 Co) and 120 oxygen sites. For every symmetrically inequivalent oxygen site, the O K-edge spectrum was computed with the excited-electron-and-core-hole (XCH) method as implemented in VASP 6.4.2, using the R2SCAN meta-GGA functional and GW-type PAW data sets; spectra were aligned via their total-energy-difference excitation onsets. The dataset contains 923 site-resolved O K-edge spectra together with the ground-state reference structures and the VASP input files (INCAR, POSCAR, KPOINTS) for both the ground-state and the core-hole calculations. The site-resolved spectra encode the spectral signatures of the individual local oxygen environments and can be combined into per-structure spectra or exploited further, for instance for machine-learning-assisted spectral interpretation. Helper scripts and a structure index are included. POTCAR files are not distributed (VASP license); experimental reference spectra used in the associated manuscript are not included and should be obtained from the original publication.</p>