Density Functional Theory calculations are used to map out the preferential oxygen molecule adsorption sites and oxygen atom incorporation on germanium (100) surface. A comparison with primary oxidation mechanisms encountered in pure silicon and silicon germanium (100) surfaces is presented here. This study highlights opposite substrates behaviors facing oxygen molecule adsorption: 1/ surface germanium atoms move from their crystalline positions to adapt to the approaching oxygen molecule resulting in adsorbed peroxide bridge configuration, whereas oxygen molecule is fully dissociated in strand configuration on a silicon surface 2/ oxygen atoms tend to avoid each other on germanium surface whereas oxide nucleus can be observed on silicon surface even at the early steps of the oxidation process. Results show that germanium surface appears to be less reactive than the silicon substrate towards molecular oxygen species.