Understanding high temperature O2 reaction kinetics and passivation of nano-structured Si-rich materials is important for Si-containing thermal protection materials, and in situations such as incineration of Si-containing waste material. In the temperature range below ~1600 K, sublimation of Si is slow, but even for low O2 pressures, O2 etching can be rapid. Depending on temperature and O2 pressure (PO2), etching may be active, or result in passivation by growth of a thin SiO2 layer. At higher temperatures, SiO2 surface layers on Si can decompose and desorb by the reaction SiO2 + Si  2 SiO(g), and even thick SiO2 can decompose and desorb as SiO, O2, and O at temperatures above ~1800 K. Silicon carbide (SiC) has similar O2 etching behavior, losing SiO and CO/CO2, and passivating due to SiO2 layer growth under the right conditions. We previously reported a study of sublimation and O2 etching of individual Si and SiC nanoparticles (NPs), and interestingly, it was found that under etching conditions, the NPs eventually passivated, even though the temperature (TNP) and PO2 conditions examined were in the range where bulk Si passivation is not observed. This difference in passivation was attributed to the limited volume of the NPs, affecting surface-to-subsurface oxygen diffusion. Silica NPs, where the NP volume is initially oxygen saturated, provide a different perspective on the etching and passivation behavior for Si-rich materials, and particularly on the role of subsurface oxygen, and that is the focus of this work.