Nanotechnology is now playing an emerging role in green synthesis in agriculture as nanoparticles (NPs) are used for various applications in plant growth and development. Copper is a plant micronutrient; the amount of copper oxide nanoparticles (CuONPs) in the soil determines whether it has positive or adverse effects. CuONPs can be used to grow corn and wheat plants by combining Bacillus subtilis. In this research, CuONPs were synthesized by precipitation method using different precursors such as sodium hydroxide (0.1 M) and copper nitrate (Cu(NO3)2) having 0.1 M concentration with a post-annealing method. The NPs were characterized through X-ray diffraction (XRD), scanning electron microscope (SEM), and ultraviolet (UV) visible spectroscopy. Bacillus subtilis is used as a potential growth promoter for microbial inoculation due to its prototrophic nature. The JAR experiment was conducted, and the growth parameter of corn (Z. mays) and wheat (Triticum aestivum) was recorded after 5 days. The lab assay evaluated the germination in JARs with and without microbial inoculation under CuONP stress at different concentrations (25 and 50 mg). The present study aimed to synthesize CuONPs and systematically investigate the particle size effects of copper (II) oxide (CuONPs) (< 50 nm) on Triticum aestivum and Z. mays. In our results, the XRD pattern of CuONPs at 500 °C calcination temperature with monoclinic phase is observed, with XRD peak intensity slightly increasing. The XRD patterns showed that the prepared CuONPs were extremely natural, crystal-like, and nano-shaped. We used Scherrer’s formula to calculate the average size of the particle, indicated as 23 nm. The X-ray diffraction spectrum of synthesized materials and SEM analysis show that the particles of CuONPs were spherical in nature. The results revealed that the synthesized CuONPs combined with Bacillus subtilis used in a field study provided an excellent result, where growth parameters of Z. Mays and Triticum aestivum such as root length, shoot length, and plant biomass was improved as compared to the control group.