Silicon nanoparticles are an emerging and promising material in many fields including electronics, catalysis, and biomaterial engineering. Synthesis in the gas phase via aerosol routes allows tuning and engineering material features such as primary particle size distribution, agglomerate size distribution, crystallite size, and morphology. Proper control of these features as well as post-synthesis processing such as surface oxidation is very relevant for making the nanoparticles chemically stable. Therefore, a kinetic expression for determining the extent of oxidation in silicon nanoparticles is necessary. Significant work has been devoted to understanding the kinetics of monocrystalline silicon, and generally accepted models such as the one by Deal and Grove provide a very accurate description of bulk silicon oxidation. However, these models are not as accurate for the first hundreds of angstroms of the oxidation. While this might be acceptable for bulk wafers, it has critical results for silicon nanoparticles with diameters around such range. Furthermore, many applications involve silicon nanoparticle aggregates with a shape far away from perfect spheres. For this reason, in this work, we propose an expression for the parabolic oxidation kinetics of polycrystalline silicon nanoparticle aggregates at 1000 °C with surface area in the range of 3 to 19 m²/g. and crystallite sizes from 50 to 70 nm. The activation energy of the process is also reported to be linked to the crystallite size.