Okta
To set up the Okta OIDC provider, first visit Okta Developer and sign up to get a developer account.
Navigate to the Applications > Applications tab, click Create App Integration, select a Sign-in method of OIDC - OpenID Connect, and choose an Application type of Web Application, then click Next.

Enter the Sign-in redirect URIs, such as https://door.casdoor.com/callback.

In the Assignments section, define the type of Controlled access for your app and then click Save to create the app integration.
Now you will have the Client ID, Client secret, and Okta domain.

Add an Okta OAuth provider in the Casdoor dashboard by entering your Client ID, Client secret, and Domain.

Note that the Domain is not just the Okta domain; /oauth2/default should be appended to it.
Visit Okta docs on authorization servers to get more details.
Now you can use Okta as a third-party service to complete authentication.