While Yahoo! uses OpenID as a mechanism for authenticating users, Yahoo! also uses OAuth for authorizing permission to potentially sensitive user data.
To ease the process of using both OpenID and OAuth, Yahoo! now supports an extension to OAuth that allows the OpenID Relying Party to also include an OAuth approval request within an OpenID authentication request. Yahoo! is a Combined Provider, which means that it serves both as an OpenID Identity Provider (OP) and an OAuth Service Provider.