Solid-state nuclear magnetic resonance (NMR) spectroscopy was used to characterize the chemical changes induced by thermal air oxidation of biomass chars. Post-pyrolysis air oxidation (PPAO) was applied to anoxically-prepared maple-wood and pecan-shell chars at PPAO temperatures ranging from 300 to 600 °C for up to 40 min. The 13C NMR data showed that PPAO treatment introduced oxygen functionalities into aromatic rings, primarily C–O and C=O, but also carboxyl groups (COO) identified after spectral editing. The concentration of COO was relatively low (<3% of all C), consistent with potentiometric titration, and reached a discernable maximum slightly above 300 °C. The COO enhancement from PPAO was lower than that observed after wet-chemical oxidation with HNO3 or ammonium persulfate. Concentrations of the C–O and C=O groups reached a maximum at PPAO temperatures between 350 and 400 °C, and clearly decreased at higher temperatures. These oxygen-containing functional groups were shown to generally increase with increasing PPAO time. Insight into the types and concentrations of oxygen functional groups induced by exposure of biomass chars to hot air has major implications for an understanding of the interaction of char with nutrients, natural organic matter, pollutants, and microbes, as well as electron transfer processes in soil.