OpenID Connect + Sign-In from OSvC Login Window?

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I'm trying to configure SSO for a client. They use an OpenID Connect based system, however, they want the sign-in process to begin in the OSvC login page. 

In the documentation, these are two different sections: "Authentication Using an External Identity Provider on the Oracle Service Cloud Login Window" and "Use OpenID-based External Identity Providers".

I've done the OpenID configuration and seem to be getting very close to signing in, but it fails when the IdP redirects back to the site after authorizing. It sends a URL that looks like this (I'm masking the customer name):

https://xxxxxxxx.custhelp.com/cgi-bin/xxxxxxx.cfg/php/sso/oidc/idp/oidcs.php?code=<some