We present an experimental study combined with first-principle methods and the chemical approach to characterize NiO surfaces. The comparison of two simulated surfaces, the apolar (100) and the polar (111) surfaces, allows to better differentiate their reactivity and their electronic properties. The redox chemistry upon the adsorption of different gases such as H-2 and H2O on pure and defective surfaces is also considered. The synthesized NiO nanoparticles obtained from triethylamine have plate-like morphologies with a crystalline size of 11 nm and the (111) surface, being the basal plane. This dominant surface is the most reactive one, it is hydroxylated at room conditions. Theoretical results confirm that the (111) surface is also likely to be hydroxylated under catalytic conditions for alkane oxidative dehydrogenation (ODH). The (001) surface at the opposite does not adsorb H-2 and is dehydroxylated. This apolar surface is not expected to be reactive. The results enable an interpretation of the high reactivity of the catalyst regarding morphological data provided by SEM (scanning electron microscopy) and HRTEM (high resolution transmission electron microscopy).