High-entropy materials are a new class of materials that have lately been developed and applied to a variety of potential fields. Here we present that electrochemical activation facilitates the application of high-entropy molybdates (HEMo) as oxygen evolution catalysts in alkaline media. The activated HEMo delivers 10 mA cm-2 at a lower overpotential by 75 mV than the pristine HEMo and exhibits the Tafel slope as low as 45 mV dec � 1. In particular, no significant decrease in activity is observed during 24 h of electrolysis. Operando Raman spectroscopy reveals that NiOOH is the dominating active phase under OER conditions for the pristine HEMo, while FeOOH is predominant for the activated HEMo. Therefore, the origin of the enhanced OER performance is likely attributed to the change in active sites from Ni to Fe as well as the increase in electrochemical surface area after activation. This work provides an application of HEMo to electrocatalysis, a design approach for the synthesis of nanoparticles containing multiple cations, and valuable insights into FeOOH as an active OER catalyst.