Old Corrugated Containerboard (OCC) pulp is an abundant source of recovered lignocellulosic material. However, its morphological and chemical heterogeneity presents challenges for upgrading. This study investigates the integration of fiber fractionation followed by oxygen delignification to increase OCC homogeneity by enhancing cellulose content. The pretreated OCC was dissolved in [DBNH][OAc] to assess its potential wet spinnability based on temperature dependence and rheological behavior. Fractionation into short- and long-fiber fractions was accomplished using a Pulmac Masterscreen with fine slots of 381 or 152 µm. Fractionation effectively removed non-cellulosic components, reducing ash, fines, and hemicellulose content, while also enhancing the efficiency of subsequent oxygen delignification. The fractionated OCC samples were oxygen delignified using sodium hydroxide charges of 6 and 8%. The oxygen-delignified OCC long-fiber fraction retained higher intrinsic and cellulose-corrected viscosity and yield while achieving a greater degree of delignification and ash reduction relative to the corresponding short-fiber fraction obtained from the same screen. This remained true for the high-yield OCC sample obtained from fiber fractionation using the larger slotted screen. Increasing the NaOH charge from 6 to 8% reduced pulp intrinsic and cellulose-corrected viscosity sufficiently to produce dopes with negligible filtration complications and a storage-to-loss modulus crossover frequency (ωc) near 65–75 °C, within the practical temperature range for wet-spinning [DBNH][OAc] dopes. These results indicate that the combination of mechanical fiber fractionation and oxygen delignification is sufficient to achieve a workable dope for [DBNH][OAc] dissolution and subsequent dry-jet wet spinning of OCC.