The Oracle Database Appliance is optimised for Oracle Database and is engineered and supported by Oracle. In order to make use of a stable and supported environment it is best practice to keep the ODA updated with the most recent patches and firmware provided by Oracle.
With the latest ODA firmware version 19.9 and up, Oracle has transitioned from OVM (de-supported) to KVM. This is in line with the transition Oracle has done for OCI (cloud) and Exadata X8M and brings the ODA in line with Oracle Cloud technology. (aligns ODA virtualization technology with OCI)