Scanning tunneling microscopy (STM), in conjunction with the nucleation and growth of An clusters, has been used to identify and quantify various types of defects on ordered, SiO2 thin films grown on Mo(112). On a low-defect surface, An clusters nucleate and grow at line defects with metal deposition at room temperature, whereas deposition at 850 K leads to cluster decoration primarily at step edges. On a highly defective surface, clusters nucleate and grow at point defects (oxygen vacancies and/or oxygen vacancy complexes) on the terraces, with some clusters grown on oxygen vacancy complexes remaining even after an 850 K anneal. The average cluster density for low Au coverages deposited at room temperature is identical to that obtained for the same Au coverage deposited at 850 K, consistent with complete titration of point defects by the nucleating clusters.