The degradation of LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ (NCM-811) in Li-ion batteries produces complex transition-metal oxides and binary phases that fundamentally limit cathode performance. While identifying these degradation products via X-ray absorption spectroscopy (XAS) is essential for mitigating electrochemical performance loss, interpretation remains challenging due to the structural complexity of real-world samples. In this work, we present an integrated theoretical-experimental framework combining high-resolution oxygen K-edge XAS with ab initio simulations based on many-body perturbation theory and high-throughput screening from density functional theory. We first evaluate the spectroscopic fingerprints of eight layered, spinel, and nominal rock-salt reference oxides, identifying discrepancies between the idealized single-crystal bulk phase and experimental spectra. Using high-throughput screening to analyze the oxygen p-projected density of states of 38 distinct polymorphs of NiO, CoO, and MnO, we propose that the spectral differences can emerge, among other factors, from a structural ensemble of local variations represented here by simplified structural proxies. Our work establishes a viable and rigorous computational pathway to interpret the complex landscape of degraded battery materials.