I'm afraid there's no way you can implement a "Forgot Password" functionality within your application for external OAuth providers.
Whenever a user logs in with an OAuth provider like Facebook or Google they will be redirected to their dedicated login pages. Whenever the credentials are not right, these pages will have their own "Password Recovery" functionality implemented.
The only thing you maybe could do is to offer a direct link to their recovery pages like http://www.facebook.com/recover/initiate. But as I said, that wouldn't provide a real benefit for your users because Facebook/Google's login pages will provide these links as well.