Chiral-induced spin selectivity (CISS) in chiral inorganic materials has emerged as a promising strategy for enhancing water electrolysis by generating spin-polarized hydroxyl radicals with parallel spins, which favor the formation of triplet-state oxygen (oxygen evolution reaction, OER) while suppressing undesired, randomly oriented byproducts (H₂O₂). Here, we report that manganese doping into chiral amorphous cobalt oxides (Mn:CoO_x) enhances oxygen evolution reaction (OER) performance by activating the lattice oxygen reaction pathway while preserving the intrinsic CISS effect. Through in situ Raman spectroscopy, differential electrochemical mass spectrometry (DEMS) and pH-dependent electrochemical characterization, we demonstrate that Mn doping induces a mechanistic transition from the conventional adsorbate evolution mechanism (AEM) to a lattice oxygen mechanism (LOM). Moreover, we confirm that Mn doping retains the spin-selective behavior of the chiral CoOx matrix, as evidenced by its continued suppression of H₂O₂ formation and enhanced OER performance. As a result, the optimized chiral Mn:CoOx electrocatalyst achieves a notably reduced overpotential of 266 mV at 10 mA cm⁻² (208 mV with 90% iR compensation) compared to 327 mV for undoped chiral CoO_x. Moreover, the chiral Mn:CoOx electrocatalyst achieves a current density of 11.14 mA cm⁻² at 2.0 V, maintaining stable operation at 1 A cm⁻² for over 1000 hours in an anion exchange membrane water electrolyzer (AEMWE) single cell, highlighting its strong potential for spin-selective green hydrogen production under industrially relevant conditions.