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Structural, magnetic, optical and electronic properties of GeGa 2 O 4 , CoGa 2 O 4 , GeCo 2 O 4 and CoCo 2 O 4 spinel oxides have been studied. Spinel oxides are studied by performing theoretical calculations based on density functional theory. In non-magnetic host GeGa 2 O 4 spinel oxide, significant effects of Co occupancy (at A, B and/or both sites) on the electronic, magnetic and optical behavior have been observed. Among the spinel oxides, the CoGa 2 O 4 , GeCo 2 O 4 and CoCo 2 O 4 spinel oxides show 100% spin-polarization and exhibit stable cubic structure in ferromagnetic phase. The calculated structural and magnetic parameters predict the cation distribution at A and B sites as follows: (G e 2 + ) [ G a 2 3 + ] O 4 − 2 , (C o 2 + ) [ G a 2 3 + ] O 4 − 2 , (G e 4 + ) [ C o 2 2 + ] O 4 − 2 and (C o + 2 ) [ C o 2 3 + ] O 4 − 2 for GeGa 2 O 4 , CoGa 2 O 4 , GeCo 2 O 4 and CoCo 2 O 4 spinel oxides, respectively. The electronic band gap is found to be decreased from 1.605 eV to 0.762 eV for spin↑ state while it is decreased from 1.578 eV to 0.000 eV for spin↓ state with Co occupancy in host GeGa 2 O 4 lattice. Lattice constant decreases (8.7877 Å to 8.1390 Å) as Co occupies B site and/or both (A and B) sites. For GeCo 2 O 4 spinel oxide, the calculated value of total magnetic moment (m tot = 6.12 μ B / f.u) and saturation magnetization (M S = 134.22 emu/g) is found to maximum. Optical properties reveal that the CoGa 2 O 4 , GeCo 2 O 4 and CoCo 2 O 4 spinel oxides have less threshold electronic transition energy relative to host GeGa 2 O 4 spinel oxide. Electronic, magnetic and optical characteristics show potential integration of these spinel oxides in magnetic-semiconducting devices. Image 1 • Effects of Co occupancy on different atomic sites in GeGa 2 O 4 spinel are studied. • Cubic CoGa 2 O 4 , GeCo 2 O 4 and CoCo 2 O 4 spinel oxides show 100% spin-polarization. • Co occupying A, B or both sites decreases the electronic and optical band gap. • GeCo 2 O 4 spinel oxide shows half metallic behavior with m t o t = 6.12 μ B / f.u and M S = 134.22 emu/g. • All spinel oxides show maximum absorption and optical conductivity in UV region. [ABSTRACT FROM AUTHOR]