The electronic and magnetic properties of the singly charged oxygen vacancy in undoped ZnO under biaxial strains are investigated by density functional theory calculations. A net magnetic moment (MM) of 0.561 μB is obtained for in ZnO under no strains, but the magnetic interaction between them is antiferromagnetic. The formation energy of VO and the MM induced by as well as the coupling type and strength between vary with the application of biaxial strains. Compressive strains can enhance the concentrations of VO and enlarge the MM, and strengthen the antiferromagnetic interactions between them at lower VO concentrations. However, at higher VO concentrations, the coupling varies from sizable antiferromagnetic to negligible weak ferromagnetic, and then becomes paramagnetic with the increase of compression. Antiferromagnetic results are further confirmed by the local density approximation with Hubbard U (LDA + U) calculations.