Waste cooking oil, a known environmental pollutant, has been used as a precursor for the synthesis of onion-like nanocarbons (OLNCs) using flame pyrolysis. The OLNCs were added to TiO₂ to form TiO₂/OLNC nanocomposites through hydrothermal treatment. The TiO₂/OLNCs ratio was varied by increasing the mass of the OLNCs (10, 20, 30, and 50 mg), while the mass of TiO₂ (100 mg) was kept constant at C to TiO₂ molar ratios of 1:2, 1:4, 1:6, and 1:10, respectively. The surface area of the photocatalysts increased with an increase in the mass of OLNCs. The nanocomposites were applied in the photocatalytic degradation of methyl orange. The photocatalysts showed a degradation efficiency trend of TC-10 > (99.9%) TC-20 > (90%) TC-30 > (81%) TC-50 > (70%) TiO₂ (44%) in 120 min. A similar trend was observed from the first-order kinetic rate data. The degradation efficiency of methyl orange was improved by adding 5% H₂O₂ (99.9%) in 30 min. The OLNCs were responsible for increased photocatalytic activity due to a high adsorption efficiency compared to pure TiO₂. The OLNCs acted as an electron acceptor, while the TiO₂ acted as an electron donor. The enhanced catalytic behavior was achieved by hindering the recombination of e⁻/h⁺ in the composite and increasing the adsorption capability of TiO₂.