High-valence metals (such as, Mo, W, Zr and Nb) have recently been reported that promote oxygen evolution reaction (OER) activity of the 3d-transition metal electrocatalysts. These high-valence metals play a key role on surface self-reconstruction process and stabilizing the low-valence active sites. However, further understand their effect on the OER mechanism is challenging, especially in amorphous electrocatalysts with complicated structure. Here, we integrate high-valence Mo with 3d metals (Fe, Co and Ni), fabricating Fe34-xCo25Ni25MoxP8B8 amorphous electrocatalysts by the melt-spinning method. We employ in-situ Raman spectroscopy to characterize the species evolution during OER, and find that the higher OER performance is originated from a bifunctional mechanism involves two catalytic sites (NiOOH and FeOOH) enabled by high-valence Mo dissolution. Benefit from this, the Fe34-xCo25Ni25MoxP8B8 with Mo show higher OER performance compared with Fe34-xCo25Ni25MoxP8B8 without Mo. For 3d-metal OER electrocatalyst, this study provides a new understanding on the effect of high-valence metals.