“Out-of-pocket” expenditures (also known as copays or deductibles) are the portion of household healthcare expenditures not covered by compulsory basic insurance or supplemental insurance. Measuring their volume is essential to managing the sickness-insurance system. At macroeconomic level, they are monitored by the health accounts. However, they are bound to vary significantly from person to person. Unfortunately, there are no individual sources for observing them directly at a detailed level. Our article describes an approach designed to fill this gap: the OMAR model (French acronym for “Microsimulation Tool for Analyzing Out-of-Pocket Expenditures”). The model breaks down personal expenditure between the three financing entities: the government sickness-insurance agency (Sécurité Sociale), the supplemental-insurance provider, and the patient. We use two sources for this. The first, called EPAS-SPS, matches data from the IRDES Health and Social Protection Survey with administrative data from the National Sickness-Insurance Fund (CNAM). The second source is a DREES survey of supplemental-insurance providers. It allows us to identify the main characteristics of the policies they offer. We use various imputation methods to combine and complete information from the two sources. The model has begun to be used for applied studies such as the analysis of the redistribution performed by the healthcare system by age and income group. By contrast, the goal of this article is essentially methodological. We describe the model's main components, in particular the imputation procedures chosen to complete the data set with points not directly observable in either source. Robustness tests give an idea of the quality of results obtained.