Idea Summary Support OCI Instance principal Authentication support in web credentials
Use Case For APEX in OCI to make internal OCI services API call
Resource Principals are not supported for APEX REST Data Sources, APEX_WEB_SERVICE or Web Credentials. The main reason is that these are granted to the very PDB, which does not fit well with the APEX isolation model (multiple isolated workspaces within a PDB). Other reasons are related to the way how APEX uses the UTL_HTTP package, which also does not play well with resource principal.
So, to use a resource principal, you'd need to work with PL/SQL and the DBMS_CLOUD package. For a REST Data Source you'd need to use OCI API keys for authentication.