The synergistic activation of metal and lattice oxygen in transition metal-based catalysts through adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM) is regarded as one of the key strategies for boosting oxygen evolution reaction (OER) activity and stability, but still remains challenge. Herein, leveraging an in-situ electrochemical activation, we reveal that the designed bimetallic phosphide heterojunction is dynamically activated into coexisting phosphides and hydroxides active phases, each following the AEM and LOM pathways respectively. Specifically, Ni atoms at the dense NiFeP@FeP heterointerfaces effectively tune the rate-determining step (RDS) (OH* to O*) energy barrier under the AEM pathway, while the interface interaction in NiFeP-NiFe layer double hydroxide (LDH) weakens the metal–O bond and lowers the d-band center of metal sites, enabling effective LOM pathway. In-situ spectroscopy combined with ¹⁸O-isotopic labelling and differential electrochemical mass spectrometry (DEMS) measurements revealed dynamic reconfiguration and both involved adsorbed/lattice oxygen signature products in the oxygen release process, affirming the synergy of AEM-LOM pathway. As a result, the activated catalyst only requires overpotentials of 197/233 mV to drive current densities of 10/100 mA·cm⁻², accompanied by Tafel slope of 32 mV·dec⁻¹ and turnover frequency (TOF) value of 0.05 s⁻¹, as well as high stability for OER. This work provides insights for deeper understanding of the OER mechanism of transition metal phosphides and the design of high efficient catalysts.