In this paper, an omnidirectional reflection band of a one-dimensional periodic structure (1DPS) of Si and SiO2 with a nematic liquid crystal (5CB) as a defect layer, i.e., (Si|SiO2)3|NLC|(Si|SiO2)3, is investigated. The geometry of the 5CB molecules NLC is optimised with the help of density functional theory (DFT) using Gaussian 09 Software packageA02 and the order parameter (S) of the 5CB molecules is calculated. The S value for the 5CB molecules is found to be 0.53 for the range of the applied electric field and confirms the nematic phase of the liquid crystal at the microwave region. By taking average of the ordinary and extraordinary refractive indices of the 5CB molecules NLC, the refractive index has been calculated. The optical properties of the 1DPS of Si and SiO2 with a 5CB molecules NLC defect layer are calculated to study the optical defect mode as well as the bi-channelled omnidirectional reflection behaviour of the 1DPS (Si|SiO2)3|NLC|(Si|SiO2)3 and tuned by the electro-optic property of 5CB NLC. Such omnidirectional reflecting behaviour of the considered structure may be used to design b-channelled omnidirectional reflector and defect mode filter and others.