OmniAuth.config.logger = Rails.logger

本番でOAuth認証したら無言でFailureに誘導される、なんて時に助かる設定。
READMEにも記載されています。

さっきまで何も教えてくれなかったログに、

I, [2015-02-27T02:45:34.384686 #21255]  INFO -- : (google_oauth2) Callback phase initiated.
E, [2015-02-27T02:45:34.627523 #21255] ERROR -- : (google_oauth2) Authentication failure! invalid_credentials: OAuth2::Error, {"errors"=>[{"domain"=>"usageLimits", "reason"=>"accessNotConfigured", "message"=>"Access Not Configured. The API (Google+ API) is not enabled for your project. Please use the Google Developers Console to update your configuration.", "extendedHelp"=>"https://console.developers.google.com"}], "code"=>403, "message"=>"Access Not Configured. The API (Google+ API) is not enabled for your project. Please use the Google Developers Console to update your configuration."}: 
{
 "error": {
  "errors": [
   {
    "domain": "usageLimits",
    "reason": "accessNotConfigured",
    "message": "Access Not Configured. The API (Google+ API) is not enabled for your project. Please use the Google Developers Console to update your configuration.",
    "extendedHelp": "https://console.developers.google.com"
   }
  ],
  "code": 403,
  "message": "Access Not Configured. The API (Google+ API) is not enabled for your project. Please use the Google Developers Console to update your configuration."
 }
}

という親切なメッセージが表示されるようになります。（正直、デフォルトでこうしといてよ、という気も。）

Google+ APIの有効化、忘れがち。