Overview of Functions:
Oracle Functions is a fully managed, multi-tenant, highly scalable, on-demand, Functions-as-a-Service platform. It is built on enterprise-grade Oracle Cloud Infrastructure and powered by the Fn Project open source engine. Use Oracle Functions (sometimes abbreviated to just Functions) when you want to focus on writing code to meet business needs.
The serverless and elastic architecture of Oracle Functions means there's no infrastructure administration or software administration for you to perform. You don't provision or maintain compute instances, and operating system software patches and upgrades are applied automatically. Oracle Functions simply ensures your app is highly-available, scalable, secure, and monitored. With Oracle Functions, you can write code in Java, Python, Node, Go, and Ruby (and for advanced use cases, bring your own Dockerfile, and Graal VM).
You can invoke a function that you've deployed to Oracle Functions from:
- The Fn Project CLI.
- The Oracle Cloud Infrastructure SDKs.
- Signed HTTP requests to the function's invoke endpoint. Every function has an invoke endpoint.
- Other Oracle Cloud services (for example, triggered by an event in the Events service) or from external services.
so You can then deploy your code, call it directly or trigger it in response to events, and get billed only for the resources consumed during the execution.
Invoking Oracle Functions from Other Oracle Cloud Infrastructure Services:
You can invoke functions in Oracle Functions from other Oracle Cloud Infrastructure services. Typically, you'll want an event in another service to trigger a request to invoke a function defined in Oracle Functions.
This functionality is currently available in:
1. The Events service. For more information, see Overview of Events.
2.The Notifications service. For more information, see Notifications Overview. For a scenario, see Scenario A: Automatically Resize VMs.
3.The API Gateway service. For more information, see Adding a Function in Oracle Functions as an API Gateway Back End.
4.The Oracle Integration service, using the OCI Signature Version 1 security policy. For more information, see Configure Oracle Integration to Call Oracle Cloud Infrastructure Functions with the REST Adapter in Using the REST Adapter with Oracle Integration.

so OCI Registry services cannot trigger your functions directly
References:
https://docs.cloud.oracle.com/en-us/iaas/Content/Functions/Tasks/functionsintegratingwithother.htm
https://docs.cloud.oracle.com/en-us/iaas/Content/Functions/Concepts/functionsoverview.htm
https://blogs.oracle.com/cloud-infrastructure/announcing-notifications-triggers-for-serverless-functions