Herein, the synthesis, structural characterization, and magnetic characteristics of ZnO nanoparticles were studied. Pure wurtzite nanoparticles were grown into an alkali salt environment with a size of 10.37 ± 0.4 nm, determined by transmission electron microscopy. X-ray diffraction, Rietveld refinement, Raman, UV–Vis, photoluminescence spectroscopy, and atomic-resolution electron microscopy were performed to verify the structure and purity of the produced nanoparticles. Additionally, magnetization versus temperature measurements showed a low-temperature peak related to an antiferromagnetic transition of oxygen. This peak changes its shape with the rise of the magnetic field, and it practically disappears at 10,000 Oe. In addition, we observed that the transition to diamagnetism occurred at a lower temperature. Magnetization as a function of magnetic field curves presents the coexistence of antiferromagnetic and ferromagnetic behavior at 2 K. Meanwhile, diamagnetism was observed at magnetic fields over 1500 Oe and ferromagnetism between ± 1500 Oe at 300 K; these features were related to the vacancies of O and Zn.