Heterogeneous data formats complicate unified analysis in multisite clinical studies. Standardizing data in the Observational Medical Outcomes Partnership (OMOP) Common Data Model (CDM) requires Extract-Transform-Load (ETL) processes, which are complex and time-consuming to develop, especially with different source data specifications. The aim of our work is to develop a generalized, metadata-driven ETL process to transform Fast Healthcare Interoperability Resources (FHIR) into OMOP CDM. In this paper, we present first results of the developed metadata-driven ETL process on the example of two different Patient FHIR specifications.