Just a quick tip:

If you’re trying to use Google Oauth 2.0 to authenticate users in your MVC 5 application, start with this tutorial:

Code! MVC 5 App with Facebook, Twitter, LinkedIn and Google OAuth2 Sign-on

When you try to log in using a Google account, you’ll get an access_denied error back from Google. To resolve the issue, read this blog post:

Changes to Google OAuth 2.0 and updates in Google middleware for 3.0.0 RC release

In a nutshell, you need to enable Google+ API access for your application from the Google Developers Console.

More info in this Katana Project issue.