► Pure zinc oxide (ZnO) particles were fabricated by a simple aqueous solution route. ► Hydrogen peroxide (H2O2) treatment was used to reduce the negative influences of oxygen vacancies and promote the proportion of oxygen interstitials. ► There is an enhancement in the ferromagnetism of ZnO after H2O2 treatment. ► Oxygen interstitials may have some close connections with the ferromagnetism of ZnO. Pure zinc oxide (ZnO) particles fabricated by a simple aqueous solution route were treated with hydrogen peroxide (H2O2) solution. X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed the homogeneous growth of particles throughout the substrate with the wurtzite phase. Magnetic properties were measured and a correlation between immersion time and ferromagnetism (FM) was observed. X-ray photoemission spectroscopy (XPS) and photoluminescence (PL) were then performed to investigate the influence of hydrogen peroxide (H2O2) treatment on oxygen-related defects. It is revealed that oxygen interstitials (IO) may be considered to be playing an important role in tailoring the magnetic properties of ZnO.