The toxicity of nanomaterials can sometimes be attributed to photogenerated reactive oxygen species (ROS), but these ROS can also be scavenged by nanomaterials, yielding opportunities for crossover between the properties. The morphology of'nanomaterials also influences such features due to defect-induced properties. Here we report morphology-induced crossover between pro-oxidant activity (ROS generation) and antioxidant activity (ROS scavenging) of MgO. To study this process in detail, we prepared three different nanostructures of MgO (nanoparticles, nanoplates, and nanorods) and characterized them by HRTEM. These three nanostructures effectively generate superoxide anions (O-2(center dot-)) and hydroxyl radicals ((OH)-O-center dot) at higher concentrations (>500 mu g/mL) but scavenge O-2(center dot-) at lower concentrations (40 mu g/mL) with successful crossover at 200 mu g/mL. Nanorods of MgO generate the highest levels of O-2(center dot-), whereas nanoparticles scavenge O-2(center dot-) to the highest extent (60%). Photoluminescence studies reveal that such crossover is based on the suppression of F2+ and the evolution of F+, F-2(+), and F-2(3+) defect centers. The evolution of these defect centers reflects the antibacterial activity of MgO nanostructures which is initiated at 200 mu g/mL against Gram-positive S. aureus ATCC 29737 and among different bacterial strains including Gram-positive B. subtilis ATCC 6633 and M. luteus ATCC 10240 and Gram-negative E. coli ATCC K88 and K. pneumoniae ATCC 10031. Nanoparticles exhibited the highest antibacterial (92%) and antibiofilm activity (17%) against B. subtilis ATCC 6633 in the dark. Interestingly, the nitrogen-centered free radical DPPH is scavenged (100%) by nanoplates due to its large surface area (342.2 m(2)/g) and the presence of the F-2(+) defect state. The concentration-dependent interaction with an antioxidant defense system (ascorbic acid (AA)) highlights nanoparticles as potent scavengers of O-2(center dot-) in the dark. Thus, our findings establish guidelines for the selection of MgO nanostructures for diverse therapeutic applications.