Crystalline ZnO and ZnO2 nanoparticles (NPs) were prepared in one-pot oil-in-water (O/W) microemulsion media at room temperature by adding an aqueous solution of sodium hydroxide (followed by H2O2 in the case of synthesis of ZnO2) to the O/W microemulsion containing a Zn(II) organometallic precursor dissolved in the oil phase. After calcination of ZnO2 NPs, ZnO NPs were also obtained, and their characteristics were compared with those of ZnO NPs prepared directly in O/W microemulsion. The materials were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy.