Molecular dynamics simulations of ZnO nanowires under tensile loading were performed and compared with simulations of TiO2 wires to present size-dependent mechanical properties and super ductility of metal oxide wires. It is shown that while large surface-to-volume ratio is responsible for their size effects, ZnO and TiO2 wires displayed opposite trends. Although the stiffness of both vires converged monotonically to their bulk stiffness values as diameter increases, bulk stiffness represented the upper bound for ZnO nanowires as opposed to the lower bound for TiO2 wires. ZnO nanowires relaxed to either completely amorphous or completely crystalline states depending on wire thickness, whereas a thin amorphous shell is always present in TiO2 nanowires. It was also found that when crystalline ZnO nanowires are stretched, necking initiated at localized amorphous regions to eventually form single-atom chains which can sustain strains above 100%. Such large elongations are not observed in TiO2 nanowires. Using the analogy of a clothesline, an explanation is offered for the necessary conditions leading to super ductility.