Defect sites on oxide surfaces play a dominant role in surface chemistry. The direct atomistic study of these sites is important but very difficult. We have mimicked the adsorbate-defect interaction by a dynamic force microscope tip measuring the interaction with a color center (F(0)) on the MgO(001) surface. The experimental findings, complemented by density functional theory calculations, show a highly attractive adsorbate-defect interaction and a charge transfer at a critical distance.