The NCCN EHR Oncology Advisory Group was formed to address the high variance in oncology history documentation within EHR systems. Specifically, there is a lack of standardization in the names of oncology history categories across institutions. Moreover, no synonym list or mapping currently exists between the varied category names. Furthermore, oncology history documentation is often inconsistent, incomplete, and not updated appropriately. The NCCN EHR Oncology Advisory Group has established best practices and is working with EHR vendors to implement and enhance tools for documentation of oncology history. This ontology is a result of the work of the group, and provides users with a list of oncology history categories and synonyms. It is our sincere hope that this work can form the basis of improving standardization and interoperability of oncology history documentation within EHR systems.