Rydberg-valence (RV) mixing and doublet-quartet (DQ) interchannel coupling in the core-excited states of triplet open-shell O2 are experimentally and theoretically unveiled through the resonant inelastic x-ray scattering around the O 1s→σ∗u resonance. Ab initio calculations explain complicated excitation energy dependence of the spectral profile by the interplay of the DQ interchannel coupling under the presence of the RV mixing. Complicated intermediate states with valence and/or Rydberg character are successfully analyzed through the filtering of pure valence and pure Rydberg final states.