A one-pot and green synthesis methodology was successfully designed to prepare nitrogen-doped carbon (NC) cohered Fe₃O₄ nanoparticles with rich oxygen vacancies (Fe₃O₄-OVs/NC). The preparation was achieved via cold-atmospheric-pressure air plasma using Fe₂O₃ nanoparticles as the only precursor, and pyridine as the carbon and nitrogen source. Systematic characterization results of the as-prepared Fe₃O₄-OVs/NC confirmed the transition from Fe₂O₃ to Fe₃O₄, along with the generation of oxygen vacancies, while preserving the original needle-like morphology of Fe₂O₃. Moreover, the results indicated the formation of the NC attaching to the surface of the formed Fe₃O₄ nanoparticles with a weight percent of ~13.6%. The synthesized nanocomposite was further employed as a heterogeneous Fenton catalyst to remove phenol from an aqueous solution. The material has shown excellent catalytic activity and stability, demonstrating a promising application for wastewater treatment.