The effectiveness of onion peel extract, (OPE), for carbon steel in one molar (1 M) hydrochloric acid solution was illustrated utilizing weight loss (WL) techniques, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and surface morphological examination. Also, this process was characterized using Fourier transform infrared (FT-IR), energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), atomic force microscopy (AFM), and scanning electron microscope (SEM). The protective efficiency rises as OPE concentration rises and decreases with corrosive solution temperature, according to weight loss measurements. Polarization curves display that OPE behaves as a mixed-type inhibitor in hydrochloric acid. The adsorption process mechanism is supported by EIS, which demonstrates that charge-transfer resistance rises, and double-layer capacitance falls with inhibitor concentration. OPE adhering to carbon steel follows the Langmuir isotherm, through a physical adsorption process and the inhibitor molecules were shown to have spontaneously adsorbed onto the carbon steel based on the negative values of Gibb's free energy of adsorption. The synergistic effect of copper oxide nanoparticles, CuO-NPs in combination with OPE on the inhibition corrosion of carbon steel was evaluated. The results showed that the inhibition efficiency of OPE increases in the presence of CuO-NPs due to synergistic interactions of OPE extract molecules with CuO-NPs. The electron-donating capacity of the chemical component of the OPE has been shown in theoretical studies employing quantum chemistry methods.