How to use OAuth with Active Directory

I'm not sure what OAuth gives you that can't be accomplished using other means. WCF + Rest works very well with claims based authentication bundled in WIF.

Since WCF implies you're using ASP.NET I recommend using Windows Identity Foundation (WIF) on the server side. Check out this ebook for more info.

Next you need a way to expose AD to your app. You can use ADFSv2 which is free on Windows 2008 R2. Take a look at the SQL installation that protects from session replay attacks.

Copying and pasting the credentials from Fiddler isn't something you can completely stop. HTTPS helps, but if you really want to investigate this issue look at this Q&A on Stack Overflow.