In this paper, phosphoric acid (H₃PO₄), hydrochloric acid (HCl), and hydrogen peroxide (H₂O₂) were employed for the modification of oil-based drill cutting ash (OBDCA) for the first time. The adsorption of rhodamine B (RhB) on modified oil-based drill cutting ash (MOBDCA) in an aqueous medium was investigated. H₂O₂-modified OBDCA had the optimal adsorption efficiency for RhB. The physical and chemical properties of MOBDCA were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), ζ-potential, N₂ adsorption–desorption isotherm, and pore size distribution. The effect of the pH value (3–11), reaction time (10–720 min), and initial RhB concentration (10–200 mg/L) on RhB adsorption was discussed. The adsorption kinetics highly fitted with the pseudo-second-order model (R² > 0.99), which indicated that the adsorption process was dominated by chemisorption. The adsorption isotherm fitted well with the Langmuir and Freundlich models (R² > 0.97), which indicated the monolayer adsorption process and the heterogeneous adsorption process, respectively. The theoretic adsorption capacity (50 mg/g) for RhB was achieved by H₂O₂-modified OBDCA. This paper provides a promising method of resource utilization of OBDCA to treat organic pollutants.