This study explains how nickel oxide (NiO) nanoparticles are made using a plasma jet method at normal air pressure, using a direct current (DC) power supply set to a steady voltage of 13 kV. The properties of the plasma were studied using optical emission spectroscopy (OES) while changing the flow rates of argon gas to 0.25, 0.75, 1.75, and 2.25 L/min over a period of 6 minutes. NiO thin films were deposited on glass substrates and annealed at 270 °C. UV-visible spectroscopy confirmed the optical properties of the NiO nanoparticles, revealing a marked decrease in the energy band gap from 4 eV to 2.5 eV with an increase in gas flow rate. X-ray diffraction (XRD) analysis demonstrated that the films possessed a polycrystalline structure with a cubic crystal system. Atomic force microscopy (AFM) analysis showed that as the argon flow rates increased, the surface became rougher and the average size of the particles got smaller. This work shows that the plasma jet system is a new, fast, environmentally friendly, and scalable way to make high-purity NiO nanoparticles that have better optical properties. These results suggest promising applications of NiO nanoparticles in photovoltaic devices, solar cells, and other optoelectronic devices.