A heterophase method to prepare hollow and/or porous crystalline nanoparticles of metal oxides at room temperature is presented, taking cerium(IV) oxide and gamma-iron(III) oxide (i.e., maghemite) as representative cases. The crystallization begins at the oil-water interface in aqueous nanodroplets of the precursor in inverse (water-in-oil) miniemulsion systems, and it may continue toward the inner part of the droplets. A poly(styrene-b-acrylic acid) block copolymer is used as a structuring agent because the ability of the carboxylic groups to bind metal ions improves the inorganic shell formation. A precipitating base is added from the continuous phase, generating hydroxide species at the interface that begin the crystallization. We analyze the effects of the synthetic parameters in terms of colloidal stability and morphology of the resulting materials. In the case of maghemite samples, the prepared dispersions of hollow particles present a distinct magneto-fluidic behavior.