The reversibility of a moderate thermal treatment (TT) at 120 °C on modern and historical oak wood samples was investigated by 1H nuclear magnetic resonance (NMR) relaxometry, equilibrium moisture content (EMC) determination, and measurements of volumetric deformations. The EMC values showed full reversibility after conditioning the samples at 65% relative humidity (RH) for one week, in agreement with previous research that utilized moderate TTs. However, 1H NMR relaxometry evidenced an irreversible effect on water mobility (i.e., molecular tumbling) for both wood samples. This result is compared to volumetric deformations that are determined by considering two reference states. When using, as a reference, the dry volume at each state, volumetric deformations are consistent with EMC values. However, when using the dry volume of the sample before TT, the volumetric deformations are found to be higher than the ones before TT for modern wood but remain lower for historical wood. The irreversible changes in water dynamics suggest that some modifications of the wood structure have not been completely recovered but are not linked to the changes in volumetric deformations. In addition, 13C solid-state NMR spectroscopy allowed the comparison of the composition between both wood samples, suggesting that their different behavior concerning volumetric deformations could be explained by a lower proportion of acetyl groups from hemicelluloses for historical wood due to its natural aging. We showed that combining macroscopic observations with molecular-level information gained from 1H and 13C NMR leads to a deeper understanding of the effects of thermal treatments and reversibility tests.