Beginner question. auth. with iOS/macOS

The idea is simple, we have our own service and we administrate our users internally. But then there is this client that manages his users with Okta and is asking if we can integrate with them. I already explored authenticating Okta users, that is quite simple. But it’s not clear to me what to do with that. I was thinking in a way to “link” the Okta user with a user in our database, but from several angles that doesn’t look like a good idea.

In short, because of the nature of our service we need the user to exist locally, I don’t see us exporting our user database somewhere else. Then, what is the recommended framework or workflow that we could use for this case?