In this work, the high quality of CdO film with a different percentage of Ga doping prepared onto optically flat glass substrates using a sol-gel spin coating progression for enhancement of its structural characteristics for various applications. X-ray diffraction results confirm that the prepared films of CdO are amorphous due to the long-range array of deposited crystals. The images of the atomic force microscopy indicated that the surface morphology of the pure and Ga-doped CdO films has high roughness values of 60–80 nm and average particle size 85–100 nm. The optical constants were calculated using the Kramer-Kronig approach. The calculated optical transmission specifies a high transmission exceeding from 77% to 85% through the non-absorbing region depending on the doping concentrations. The results confirm that the measured optical band gap is strongly influenced by doping concentrations of Ga and decreases from 2.35 to 1.72 eV. The calculated optical dispersion and non-linear optical parameters were found to be strongly affected by doping. The terahertz cut-off frequency was set for the pure and Ga-doped CdO films indicating the applicability of the prepared films as terahertz filters.