Synthesis of earth-abundant nanocatalysts for the oxygen evolution reaction (OER) has depended largely on highly diluted, batchwise wet chemical methods, leaving it a challenge to improve throughput and sustainability of the process. Herein, we demonstrate for the first time the production of Co3O4 and FexCo3–xO4 nanoparticles using vapor-fed flame aerosol synthesis (VFAS), a continuous and scalable process requiring minimal waste treatment. In 1 M KOH, the catalysts exhibit stable OER overpotentials of 295 mV at 10 mA/cm2 and Tafel slopes down to 38 mV/dec, which are comparable to the performances of wet-chemically derived benchmark (Fe-doped) Co3O4 catalysts. The high activity is attributed to ultrafine particle size of ⟨Dp⟩ = 3.1–4.4 nm and rich surface defects. Furthermore, nanostructured Co3O4 and FexCo3–xO4 films can be conveniently grown on graphite substrates by VFAS and serve as OER electrodes without further treatment. Remarkably, the morphology of the films can be easily tuned from columnar to granular by varying precursor concentration in feed gas, achieving optimal utilization of catalytic materials with a hierarchical structure consisting of elongated nanoparticulate building blocks.