The EPR spectrum of O17- on MgO has been obtained by the reaction of N2O containing 71.9% oxygen-17 isotope with trapped electrons on an MgO surface. The spectrum of the ion is characterized by g ⊥=2.042 and g ∥=2.0013 with A⊥=19.5±0.1G and A ∥=103.2±0.1G. A quadrupole interaction of Q⊥=1.3±0.2G was also determined. From the hyperfine coupling tensor the coefficients of the wavefunction of the spin state were estimated and were found to be consistent with those obtained from the g tensor. The unpaired electron is localized mainly in one 2p orbital on the O− species. These results suggest that O− is trapped at an oxide ion vacancy on the surface.