The effects of the oxygen vacancies on the microscopic potential distribution and macroscopic potential averaged over one period around the defect for silicon dioxide have been investigated via first-principles calculations. The results demonstrate that such an effect is limited to the dimensions of one cell. Detailed analysis of the planar macroscopic average potential shows that the conduction band alignment caused by the defect and its effects on the tunneling currents have been calculated. The calculations demonstrate that the relative increase in the electron direct tunneling current caused by the oxygen vacancy depends on the position of oxygen vacancy. It is also shown that the increase in the direct tunneling current caused by the oxygen vacancy exponentially decreases with increasing oxide thickness, whereas its relative increase changes little.