Artificial photosynthesis carries promise to deliver abundant clean energy for the needs of a growing population. Deep mechanistic understanding is required to achieve rational design of fast and durable water oxidation catalysts. Here we provided first evidence for a new mechanism of the O-O bond formation via radical coupling of the oxidized metal═oxo of radicaloid character (RuIV═O) and ligand based radical ([ligand-NO]+• cation radical). O-O bond formation is facilitated via spin alignment and takes place via a virtually barrier less pathway inside the single metal complex. In situ reactive intermediate conversion was monitored by mass spectrometry, resonance Raman (RR) and EPR. Computational analysis have shown that the formation of [ligand-NO]+• happens at a lower overpotential than the formation of the [RuV═O(ligand)]3+ intermediate. Overall, the presented paradigm for O-O bond formation opens new opportunities for rational catalyst design.