The effects of the lattice strain induced by neutral oxygen vacancies in ferroelectric tetragonal BaTiO(3) and KNbO(3) are investigated using ab initio simulations. We propose that an oxygen vacancy can transform from its metastable equatorial configuration to the stable axial configuration via either diffusion or rotation of the polar axis near the vacancy site by 90°. The latter mechanism, predicted to dominate in materials with slow oxygen vacancy diffusion and low formation energy of 90° domain walls, can stimulate the formation of domains with their polar axes pinned by the vacancies.