This study reports a facile strategy to prepare IrOx nanoparticle films on FTO substrates through simple spin-coating, annealing, and plasma treatment, which act as efficient electrocatalysts towards oxygen evolution reaction (OER). Typical amorphous IrOx nanoparticle film obtained by annealing at 300 degrees C and air-plasma treatment exhibits excellent OER performance in acidic electrolyte with a low overpotential of 290mV at 10mAcm(-2), a Tafel slope of 55.4mVdec(-1), and an ultrahigh mass activity of 993Ag(-1) at 1.55V, which is among the best compared to the results reported in the literature.