Using OAuth 2.0 with Windows Authentication

I am writing a Web API that must be accessed both from an internet application and from a desktop application.  Since we are moving our application to the internet eventually, the simplest way I can see doing this is to write a web front end (probably a SPA) that can be viewed in a browser window within our WPF application, as well as viewed over the internet. Since there is very sensitive data that will be obtained through this application, I thought that using OAuth 2.0 would be the best form of security.  All of our users are already in Active Directory, so I wrote a custom OAuthAuthorizationServerProvider that checks the user's credentials against AD and then returns a token if they are valid.  This works perfectly for anyone logging into our system with their username and password.  However, if this application is contained within our desktop application, we would like to have it validate automatically.  Obviously, I can get the user's identity ...