<p>Tailoring the optical gap energy and optical transitions of transition metal-containing glass is of great interest to meet increasing demands for various optoelectronics and photonics applications. This work studies the structural characteristics and optical properties of BaO-Na₂O-CoO-ZnO borate glass synthesized via the melt-quenching method. The effective impact of BaO/ZnO substitution on glass network modifications, optical basicity, Co ligand field parameters, and electronic polarizability is systematically analyzed. Structural changes are assessed through molar volume and internal structure variations, demonstrating that further BaO/ZnO substitution increases glass density and molar volume while affecting cationic distribution and interatomic separations. Optical absorption studies reveal characteristic Co²⁺ and Co³⁺ transitions, confirming the existence of Co²⁺in tetrahedral and octahedral coordination, while Co³⁺ exists in octahedral coordination only, with more BaO/ZnO replacements preference of Co²⁺ in tetrahedral coordination. Ligand field analysis indicates a decrease in tetrahedral field splitting energy (10Dq) from 3453 to 3341&#xa0;cm⁻¹ and an increase in Racah parameter (B) from 954 to 938&#xa0;cm⁻¹, signifying reduced metal–ligand interactions and increased ionic character. Optical properties, including the optical gap energy (E_g), linear refractive index (n_o), and optical susceptibility (χ³), show systematic variations with further BaO/ZnO substitution. The findings suggest that BaO-Na₂O-CoO-ZnO glass is a promising candidate for ultraviolet shielding, optical filters, and nonlinear optical applications.</p>