We discuss the capability of core-level resonant X-ray emission spectroscopy (RXES) as a tool to investigate the electronic states of highly-correlated electron systems. Particular attention is paid to how the many-body excitations can be observed in the oxygen 1s resonant spectra in some cuprates. We emphasize the importance of resolving the polarization of incident and emitted X-rays in detecting the “multiple-copper-site” effects.