Lignin is an important macromolecule present in biomass, and its structure contains a great diversity of aromatic rings and organic chains that can potentially be converted into smaller molecules with high added value. Several methods for the derivatization of lignin are considered in the literature; however, they come up against the high recalcitrance of lignin. As an alternative, oxidation of α-hydroxyls, as a preliminary step, facilitates the breakdown of its polymeric chain. An efficient oxidative method was developed in this work, consisting of the oxidation of banana organosolv lignin via atmospheric oxygen gas in the presence of Co₃O₄ as a heterogeneous catalyst at a temperature of 80 °C and ambient pressure. The ideal concentration of the catalyst is 1% (mol/mol), which after 54 hours of reaction was sufficient for the maximum growth of the intensity of the carbonyl band at 1710 cm⁻¹ (by FTIR), increasing the transmittance by 0.32%. A decrease in intensity by 0.31% of the band at 3400–3430 cm⁻¹ is also observed (hydroxyls), confirming the effectiveness of the method and the conversion capacity of the catalyst. The non-appearance/intensification of other bands is attributed to the good selectivity of Co₃O₄.