© 2016 American Chemical Society. Multicomponent metal-oxide nanoparticles are appealing structures from applied and fundamental viewpoints. The control on the synthetic parameters in colloidal chemistry allows the growth of complex nanostructures with novel morphologies. In particular, the synthesis of biphase metal-oxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of H2O. Here we report a novel route to synthesize either fully dense or hollow core/shell metal-oxide nanoparticles (MnOx/FeOx) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O2 carried by the not properly degassed H2O or (CH3)3NO) allows the formation of hollow structures by a galvanic reaction between the MnOx and FeOx phases. In particular, the use of (CH3)3NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnOx/FeOx nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnOx/FeOx hollow nanoparticles by an intermediate step where the MnO/Mn3O4 seeds are oxidized into Mn3O4, thus allowing the Mn3+ → Mn2+ reduction by the Fe2+ ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/Mn3O4/Fe3O4 particles. Thus, we show that the critical step for galvanic replacement is the proper seed oxidation states so that their chemical reduction by the shell ions is thermodynamically favored.