We studied the structural, electrical, magnetic, and optical properties of cubic spinel oxides  (A=Cr, Mn) and  (A=Cr, Mn) using density functional theory (DFT). We employed GGA-PBE, GGA + U, GGA-PBEsol, and GGA-WC exchange-correlation functionals for our investigation. All investigated spinel compounds exhibit ferromagnetic properties. All of our investigated spinels are more favorable in the ferromagnetic configuration than in the non-magnetic configuration according to the Birch-Murnaghan equation of state. The ferromagnetic property of these spinels originates mainly from the A-site ions (Cr, Mn) because of partially filled d-orbitals, whereas B-site ions (Sc, Zn) and O atoms contribute very little. Electron charge-density and Mulliken charge analyses consistently indicate that A-site cations form covalent bonds with oxygen atoms, whereas B-site cations predominantly exhibit ionic bonding. The spin-polarized band structures, along with the density of states (DOS), confirm the half-metallic nature of the examined spinels, as demonstrated using three different functionals: GGA-PBE, GGA + U, and GGA-PBEsol, by exhibiting metallic behavior in the spin-up state and wide band gap semiconducting behavior in the spin-down state. In the spin-down state, the band gaps of the , ,and  spinels are 4.25 eV, 4.35 eV, 2.74 eV, and 2.96 eV respectively using GGA + U functionals which is greater than the values obtained from GGA and PBEsol functionals. The half-metallic behavior arises from the hybridization between Cr-d and O-p orbitals for and  spinels whereas for  and  spinels, hybridization between Mn-d and O-p orbitals is mainly responsible for half-metallic properties. Optical properties such as dielectric constants, conductivity, absorption coefficient, reflectivity, refractive index, and loss functions are also analyzed. The observed half-metallic ferromagnetism and optical analysis revealed that our materials are promising candidates for in spintronic and optoelectronic applications.