Until now, no suitable Mg/O parameters have been available to describe the passivation at the Mg anode. So far, only Cheung et al. has trained force field parameters for magnesium hydride systems or Iype et al. to describe MgSO 4 hydrates. Additional parameters for Mg/O are available in a peptide force field developed by Zhu et al. whose training set was however limited to the bulk phases of Mg and MgO. The parameters were recently reused by Yeon et al. to study magnesium aluminosilicate glasses, and the training set was extended to include additional bulk phases, heats of formation, and bond scans for the Mg/O description. The goal of the present study is to describe the initial steps of Mg anode degradation by oxygen impurities. To this end, we trained a ReaxFF reactive Mg/O force field that can accurately describe the properties of both metallic Mg and MgO. The comprehensive training set included a variety of bulk, surface, and adsorption energy values as well as diffusion barriers calculated at the level of density functional theory (DFT). In addition, we demonstrated the applicability of our force field by first completely oxidizing a Mg particle in a grand-canonical Monte Carlo (GCMC) simulation and then recrystallizing it into the thermodynamically stable rocksalt structure by simulated thermal annealing at 2000 K in a subsequent molecular dynamics (MD) simulation.