Metallic surfaces exposed to harsh, corrosive environments will form oxide scales, but alloys such as cupronickel exhibit competitive oxidation behaviors that are not yet fully understood, especially at the atomic scale. Experiments using environmental TEM show microstructure evolution on CuNi surfaces determined by elemental segregation and lattice strain during the oxide growth. Additionally, the initial alloy structure, composition, pretreatment, and service conditions affect the final scale. To understand the experimental results, we develop a Cu/Ni/O reactive forcefield (ReaxFF) to model such systems. The reference ground-truth values for energies, forces, and structural parameters are obtained using density functional theory calculations of elemental bulk systems, bulk oxides, alloyed surfaces with different surface compositions, and configurations exhibiting diffusion barriers. We employ this forcefield to understand the dynamics of surface segregation and oxide growth using a Monte Carlo approach under reducing and oxidizing conditions as a function of temperature and Ni concentration.