Atomic-level simulations equipped with a reactive force field (ReaxFF) are used to characterize atomic oxygen adsorption on a Pt(111) surface. The off-lattice Grand Canonical Monte Carlo (GCMC) calculations presented here rely solely on the interatomic potential and do not necessitate the pre-computation of surface adlayer structures and their interpolation. As such, they provide a predictive description of adsorbate phases. In this study, validation is obtained with experimental data as well as other DFT results available in the literature. The use of the GCMC technique based on a transferable potential is particularly valuable to produce more realistic systems (adsorbent and adsorbate) to be used in subsequent dynamical simulations (Molecular Dynamics) to address recombination reactions (via either Eley-Rideal or Langmuir-Hinshelwood mechanisms) on variously covered surfaces. This is demonstrated by simulations to determine the probability of Eley-Rideal recombination on Pt(111), using information provided by the GCMC computations and using the same force field parametrization. The proposed methodology is general, and does not depend on the particular crystal structure of the substrate. A preliminary simulation on Pt(533) is therefore also shown. The (533) facet is characterized by surface defects (regularly spaced steps). It was found that a much higher O coverage is determined compared to Pt(111) under the same pressure and temperature conditions. By using GCMC and Molecular Dynamics simulations, the ReaxFF force field can be a valuable tool for understanding heterogeneous catalysis on a solid surface. Finally, the use of a reactive potential is a requirement to investigate problems where dissociative adsorption occurs, as typical of many important catalytic processes.