The structural and magnetic properties of the Fe/NiO(100) interface have been theoretically studied by density-functional theory within generalized gradient approximation (GGA) and, in selected cases, GGA+U methods. By total energy calculation, we find that Fe atoms adsorb preferentially on O sites and that a chemical reduction of NiO occurs, giving rise, for adsorption of more than one monolayer, to an interface of complex chemical and structural compositions, characterized by oxygen migration toward the surface. The magnetic moments at the interface and their alignment in a ferromagnetic and/or antiferromagnetic ordering are deeply influenced by the geometrical configuration of the atoms, pointing out to an important interplay between structure and magnetic configuration. This mechanism, at small oxidation level, leads quite naturally to the appearance of uncompensated spins at the interface.