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Oxide films play a significant role in a wide range of industrial fields, mostly due to the thickness-dependent variation of their properties. Recently, it has been proposed based on the experimental study that carrier transport in CaO films proceeds via strong phonon excitations with a variable signal depending on the film thickness. In this paper, we report a detailed investigation in the frame of the density functional theory of structural and electronic properties of freestanding and Mo(100)-supported CaO films, as well as phonons therein, as functions of the film thickness and intensity of the external static electric field. Our calculations demonstrate that phonon frequencies negligibly depend on the external electric field. A small gradual increase of the energy of CaO phonons upon increase of the film thickness was found to be in line with earlier experimental findings. The effect of Mo support was observed in the systematic decrease of the energy of phonons. The applied electric field showed a minor effect on the structure of CaO films, whereas electronic properties of the oxide were significantly affected. In particular, the band gap of CaO films was found to gradually decrease with the growing intensity of the external electric field, while the effect is a more pronounced for thicker films. Overall, our paper provides innovative insights into the mechanism of electron transport and electronic properties of CaO films that might lead to new potential applications of oxide materials. peerReviewed