Efficient industrial hydrogen production via water splitting hinges on the development of highly effective oxygen evolution catalysts and a clear understanding of their catalytic mechanisms. Among various strategies, exploiting the synergistic effects of transition metals has shown great promise, although the underlying mechanisms remain elusive. Here, we investigate bi-transition metal borides, Mo₂MB₂ (M = Co and Ni), as a model system to unravel these synergistic effects and the evolution of active species during the oxygen evolution reaction (OER). Using combined in-situ and ex-situ characterization techniques, we monitor the structural and valence changes of constituent elements in real time. We find that Mo and B undergo oxidation and dissolution at the anode, initiating distinct evolutionary pathways. In Mo₂CoB₂, rapid structural collapse leads to the formation of γ-CoOOH as the active species. In contrast, Mo₂NiB₂ exhibits a more gradual surface-driven transformation, producing γ-NiOOH and Ni–O–Mo species. Chronopotentiometry testing reveals continued Mo and B dissolution, culminating in the transition of γ-phases to amorphous states, followed by recrystallization into β-phases. This study provides critical insights into dissolution-induced structural evolution, active species dynamics, and the synergistic interactions between Mo/B and Co/Ni during OER catalysis.