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We resonantly excite the $K$ series of O$^{5+}$ and O$^{6+}$ up to principalquantum number $n=11$ with monochromatic x rays, producing $K$-shell holes, andobserve their relaxation by soft-x-ray emission. Some photoabsorptionresonances of O$^{5+}$ reveal strong two-electron--one-photon (TEOP)transitions. We find that for the $[(1s\,2s)_1\,5p_{3/2}]_{3/2;1/2}$ states,TEOP relaxation is by far stronger than the radiative decay and competes withthe usually much faster Auger decay path. This enhanced TEOP decay arises froma strong correlation with the near-degenerate upper states$[(1s\,2p_{3/2})_1\,4s]_{3/2;1/2}$ of a Li-like satellite blend of the He-like$K\alpha$ transition. Even in three-electron systems, TEOP transitions can playa dominant role, and the present results should guide further research on theubiquitous and abundant many-electron ions where electronic energy degeneraciesare far more common and configuration mixing is stronger.

Observation of strong two-electron--one-photon transitions in few-electron ion