The oxidation of aluminum nanoparticles is studied with classical molecular dynamics and the Streitz-Mintmire (Streitz, F. H.; Mintmire, J. W. Phys. Rev. B 1994, 50, 11996) electrostatic plus (ES+) potential that allows for the variation of electrostatic charge on all atoms in the simulation. The structure and charge distributions of bulk crystalline alpha-Al2O3, a surface slab of alpha-Al2O3 with an exposed (0001) basal plane, and an isolated Al2O3 nanoparticle are studied. Constant NVT simulations of the oxidation of aluminum nanoparticles are also performed with different oxygen exposures. The calculations simulate a thermostated one-time exposure of an aluminum nanoparticle to different numbers of surface oxygen atoms. In the first set of oxidation studies, the overall approximate ratios of Al to O in the nanoparticle are 1:1 and 2:1. The nanoparticles are annealed to 3000 K and are then cooled to 500, 1000, or 1500 K. The atomic kinetic energy is scaled during the simulation to maintain the desired temperature. The structure and charge distributions in the oxidized nanoparticles differ from each other and from those of the bulk Al2O3 phases. In the Al1O1 simulation, an oxide shell forms that stabilizes the shape of the particle, and thus the original structure of the nanoparticle is approximately retained. In the case of Al1O0.5, there is insufficient oxygen to form a complete oxide shell, and the oxidation results in particles of irregular shapes and rough surfaces. The particle surface is rough, and the nanoparticle is deformed.