SOLVED: In my scenario, I'm calling the ORDs endpoint through the OCI API Gateway, so I have something like: OCI APEX -> OCI API Gateway -> ORDS Endpoint (On-Prem APEX on Tomcat/ORDS). So, by default, withing APEX Restful Services, when REST-Enabling the Schema where the endpoint is, the sample API methods (Resource Templates) are created with an "/" at the end (e.g. "version/". Well, hard to believe but that "/" is the cause of the error (ORA-01031) in this case. By simply removing the "/" from the name of the API methods everything works as expected. I hope this may be useful for anyone to save some hours/days of frustration. Thank you again John for this great article.

Hi John, thank you for this really great guide. I've done all the steps, trying to call an end-point from APEX on my OCI ADB and I get ORA-01031: insufficient privileges. In my case my principal is APEX_240100. The end-pont works fine from the browser or Postman, has no authentication required and it's an HTTPS address. Any idea that can help me figure it out? Thanks a lot in advance.