The relation between catalytic reactivities and metal/metal oxide ratios, as well as the functions of the metal and the metal oxides were investigated in the CO2 hydrogenation reaction over highly active Cox(CoO)1–x catalysts in operando. The catalytic reactivity of the samples in the CO2 methanation improves with the increased CoO concentration. Strikingly, the sample with the highest concentration of CoO, i.e., Co0.2(CoO)0.8, shows activity at temperatures lower than 200 °C where the other samples with less CoO are inactive. The origins of this improvement are the increased amount and moderate binding of adsorbed CO2 on CoO sites. The derivative adsorption species are found to be intermediates of the CH4 formation. The metallic Co functions as the electronically catalytic site which provides electrons for the hydrogenation steps. As a result, an abundant amount of CoO combined with Co is the optimal composition of the catalyst for achieving the highest reactivity for CO2 hydrogenation.