A green, efficient, simple, and general route has been developed to synthesize a wide variety of metal oxide nanoparticles (NPs) in large quantities at room temperature. Various metal and alloy oxide NPs like In2O3, SnO2, ZnO, Fe2O3, NiO, TiO2 ZrO2, CuO, Al2O3, and sus-304L are synthesized by anodization of metals and alloy in KCl aqueous electrolyte. Metallic wire or sheet is very rapidly and directly converted into metal oxide NPs by the anodization process, allowing mass production of oxide NPs. The produced NPs are crystalline, and their colloidal suspensions in ethanol or water remain stable for several days. The size of the synthesized NPs can be readily tuned by changing the anodizing voltage. We found that the ultrafast reaction rate caused by chloride ions plays an important role in the formation of metal oxide NPs. The produced oxide NPs are characterized using various tools. SEM and TEM results confirm the formation of various metal oxide NPs. Sus oxide NPs shows higher catalytic efficiency compared to that of commercial magnetite NPs. Our results indicate that anodization is an efficient route for the production of good quality and diverse metal as well as alloy oxide NPs on a large scale. The possible formation mechanism is presented. This approach can also be applied to other complex metals and alloy oxide NPs.