The conversion of industrial carbon wastes into functionalized graphene materials represents an attractive strategy for the sustainable production of advanced carbon nanomaterials. In this work, a graphitized coke-like waste generated during metallurgical coke production was evaluated as a precursor for the preparation of graphene oxide (GO) and amine-functionalized graphene oxide, using a commercial synthetic graphite as a reference material. Prior to oxidation, both graphites were subjected to a mild aqueous liquid-phase exfoliation (LPE) treatment employing water as the sole processing medium. Graphene oxides were subsequently prepared using a modified Hummers method and functionalized with n-octylamine through an ultrasound-assisted reaction. X-ray diffraction confirmed the successful conversion of both graphitic precursors into graphene oxide through the appearance of the characteristic (001) reflection at approximately 10–11° (2θ). Structural characterization revealed that the LPE treatment promoted partial delamination of the graphitic crystallites while preserving graphitic crystallinity. FTIR and thermogravimetric analyses indicated that LPE modified the distribution and accessibility of oxygen-containing functionalities generated during oxidation. After amination, the LPE-derived materials exhibited more pronounced spectroscopic and thermal changes, suggesting a greater participation of reactive oxygen-containing groups during functionalization. The results demonstrate that graphitized coke waste can be transformed into graphene oxide and nitrogen-functionalized graphene materials with characteristics comparable to those obtained from commercial graphite, providing a simple and sustainable route for the valorization of industrial carbon residues into value-added graphene-based material.