Using FP free energy calculations with MD simulations aided by MLFFs, we elucidated the mechanism by which melamine modification enhances ORR activity on Pt catalysts. Calculations of the OH* reduction potential revealed that, similar to Pt alloying, melamine destabilizes OH* on Pt and accelerates the OH removal process, which is the rate-limiting step of ORR. However, analyses of the electronic structure and surface atomic distances indicated that the mechanism of OH* destabilization by melamine is entirely different from the electronic effects of a d-band center downshift or strain effects caused by shortened metalmetal distances observed in alloys. FPMD analyses of solvent structure and vibrational frequencies demonstrated that melamine destabilizes OH* by disrupting the hydrogen bonding around it, thereby promoting ORR. In other words, modification with organic molecules like melamine achieves high catalytic activity through solvation effects. This advanced approach is expected to offer a new guideline for catalyst design that surpasses the traditional trade-off between activity and durability seen in alloys. In addition to elucidating the role of melamine, the results of this study demonstrate that FP calculations of redox potentials aided by MLFFs offer a method for accurately predicting free energy changes of elementary reactions at complex solid-liquid interfaces at finite temperatures, without relying on a harmonic oscillator model for structures optimized at 0 K. This method not only provides insights into the effects of organic molecules on ORR, as addressed in this study, but also opens the door for FP predictions of a wide range of electrode catalytic reaction mechanisms, such as cation effects on ORR, hydrogen evolution reaction and CO 2 reduction, and solvation effects on OER on oxide surfaces.