Although anomalous melt properties have been attributed to the presence of triply coordinated oxygen ([3]O), the presence of [3]O in covalent amorphous oxides has not been revealed experimentally; such verification is the Holy Grail in the study of the physics and chemistry of glasses. We report the first 17O NMR spectrum for amorphous Al2O3 and reveal the hidden nature of [3]O. The detailed NMR characteristics of the oxygen tricluster are distinct from those estimated for the crystalline analogs, thus indicating its unique structure. This unambiguous evidence of the presence of [3]O allows us to microscopically constrain its glass-forming ability and unique two-step crystallization paths of amorphous Al2O3 through the annihilation of glassy [3]O with multiple [5]Al species.