The digitization of healthcare has created a large amount of clinical data that is little used in research. The FHIR standard is mainly for real-time data exchange for diagnosis and treatment, not for secondary use. The study used the TermX platform to create two-way transformation rules between FHIR and OMOP CDM according to the Design Science methodology. Synthetic FHIR JSON data from five sources were tested: observation, patient, encounter, organization, and practitioner, with an emphasis on vital signs. The mapping from FHIR to OMOP CDM reached 74% coverage, from OMOP CDM to FHIR around 23%. The unmapped elements are related to the structural differences of the standards. TermX enables reusable transformations to support real data research and the European Health Data Space.[1]