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Advances in synchrotron x-ray Raman scattering (XRS) techniques enable probing of pressure-induced element-specific bonding transitions in diverse oxides beyond megabar pressures. Evolution of the electronic structures under extreme pressure involves an emergence of additional oxygen $1s$ electron excitation pattern and shifts in the electronic states toward high-energy ranges in an XRS spectrum. Structural origins of such changes and proper spectral proxy for the corresponding evolution in electronic density of states remain to be established. Here, we calculated the O $K$-edge features in the XRS spectrum for diverse crystalline ${\mathrm{SiO}}_{2}$ polymorphs, from \ensuremath{\alpha}-quartz (1 atm) to a pyrite-type structure (at \ensuremath{\sim}271 GPa), beyond the current experimental pressure limit (\ensuremath{\sim}160 GPa). The ab initio results unravel the electronic origins of the evolution in the XRS patterns, such as the emergence of the bimodal feature with decreasing oxygen-oxygen distance $({d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}) l\ensuremath{\sim}2.5 \AA{}$ and a subsequent merging of the doublet patterns with a further decrease in ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ down to $\ensuremath{\sim}2.3--2.4 \AA{}$. Ab initio simulations of the XRS spectrum of pyrite-type ${\mathrm{SiO}}_{2}$ phase at $\ensuremath{\sim}271 \mathrm{GPa}$ revealed the significant electron dispersion in $2{p}^{*}$ states and the pronounced electron density between edge-sharing oxygen atoms with ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ of \ensuremath{\sim}2.1 \AA{}, in contrast to an earlier prediction with a negligible electron density between those oxygen atoms. The pressure-induced increase in electron interactions leads to a systematic change in the edge energy at half of the spectrum area $({E}_{M})$ and the edge energy at the center of mass of the spectrum $({E}_{C})$ of XRS patterns beyond multimegabar pressures. Particularly, the ${E}_{M}$ and ${E}_{C}$ increase linearly with decreasing oxygen proximity $({d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}})$ and bulk density up to 270 GPa. The ${E}_{M}$ and ${E}_{C}$ for ${\mathrm{MgSiO}}_{3}$ phases show larger changes with varying ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ than those in ${\mathrm{SiO}}_{2}$ phases, demonstrating the effect of the electronic hybridization between Mg and O on the evolution of the XRS patterns. The strong linear correlation among ${E}_{M}$, ${E}_{C}$, and ${d}_{\mathrm{O}\text{\ensuremath{-}}\mathrm{O}}$ (and density) up to \ensuremath{\sim}270 GPa confirms that the ${E}_{M}$ and ${E}_{C}$ can be effective spectral proxies to quantify the densification of crystalline and noncrystalline oxides. The results with the statistical evaluations of the electronic density of states provide improved prospects for the prediction of the evolution in core-level excitation features of diverse complex, multicomponent oxides under multimegabar pressure conditions toward the deep lower mantle of Earth and other super-Earth bodies.