An Objective Structured Clinical Examination (OSCE) is an effective method for evaluating competencies. However, scores obtained from an OSCE are vulnerable to many potential measurement errors that cases, items, or standardized patients (SPs) can introduce. Monitoring these sources of errors is an important quality control mechanism to ensure valid interpretations of the scores. We describe how one can use generalizability theory (GT) and many-faceted Rasch measurement (MFRM) approaches in quality control monitoring of an OSCE. We examined the communication skills OSCE of 79 residents from one Midwestern university in the United States. Each resident performed six communication tasks with SPs, who rated the performance of each resident using 18 5-category rating scale items. We analyzed their ratings with generalizability and MFRM studies. The generalizability study revealed that the largest source of error variance besides the residual error variance was SPs/cases. The MFRM study identified specific SPs/cases and items that introduced measurement errors and suggested the nature of the errors. SPs/cases were significantly different in their levels of severity/difficulty. Two SPs gave inconsistent ratings, which suggested problems related to the ways they portrayed the case, their understanding of the rating scale, and/or the case content. SPs interpreted two of the items inconsistently, and the rating scales for two items did not function as 5-category scales. We concluded that generalizability and MFRM analyses provided useful complementary information for monitoring and improving the quality of an OSCE.