Conditions to increase phenolic hydroxyl groups (OH-Phe) in organosolv lignin using alkaline catalysts (NaOH and KOH) were optimized with the purpose of increasing the reactivity of lignin and to evaluate the effect on the structure of the newly generated compounds as a result of the base-catalyzed depolymerization (BCD). The lignin-derived compounds could be used for the synthesis of bio-based polymers and nanomaterials. The maximal yield of OH-Phe reached values of 5270 and 3970 μmol per gram of lignin when NaOH (8.6%, 173 °C, and 33 min) and KOH (11.7%, 174 °C, and 58 min), defined as optimal conditions for BCD, were used, respectively, while for the control lignin, OH-Phe was 2830 μmol per gram of lignin. The molecular weight (Mw) and glass transition temperature (Tg) for lignin-NaOH were lower than those of lignin-KOH and lignin-control. Both lignin derivatives had greater thermal stability at high temperatures than lignin-control. In conclusion, the results showed higher modifications in the structure of lignin as a result of the oxidation process, primarily by cleavage of the β-O-4 ether bond and variations in thermochemical properties when NaOH is used as alkaline catalyst.