<p>Although NiOOH has been widely studied as a water oxidation catalyst, its active structure and catalytic mechanism under operating conditions remain unclear. Isolating the true active phase is of great significance for further exploring the oxygen evolution reaction mechanisms in depth. Here we successfully isolated a long-lived active NiOOH phase with abundant Ni⁴⁺ and detected the presence of a stable Ni–O–O–Ni₂ phase in the bulk during the electrochemical oxygen evolution reaction. This phase spontaneously and continuously evolves oxygen in pure water at room temperature for several minutes without requiring an applied potential. Through online mass spectrometry, we demonstrate that spontaneous oxygen evolution proceeds via initial lattice oxygen coupling followed by continuous water oxidation at active sites. By studying this process, we show that the charges stored by the Ni⁴⁺ in NiOOH bulk can continuously migrate to the surface active sites to drive water oxidation. This offers guidance for the design of more advanced water oxidation catalysts and provides insights at the molecular level.</p><p></p>