I was recently looking for an OpenID Connect supporting application to do a POC for Single Sign-On using Auth0 and Okta. I came across this excellent blog from Oracle A-Team https://www.ateam-oracle.com/integrating-sso-between-apex-cloud-and-identity-cloud-service-the-easy-way
The blog explains step-by-step procedure how we can use Oracle Apex (Free) as an OpenID Connect Application and integrate it with Oracle IDCS as the IDP.
I tried the steps and it worked like a charm. I can also confirm that I have been able to successfully integrate my Apex workspace apps (they have quite a few interesting apps available like Group Calendar, Opportunity Tracker, Incident Tracker, etc) with other online Identity Providers like Okta, Auth0 and Oracle Access Manager.