How can I implement OAuth based login, just like drupal.stackexchange.com offers?
Stack Exchange uses OpenID for single-sign-on, which is not the same as OAuth.  The good news is, OpenID is built-in to Drupal core.  The downside of using the core module is your visitors must know the URL of their OpenID provider.  Most people already have at least one OpenID provider, but don't know it.

To solve this problem, you could try OpenID selector, which offers a login selector similar to SE.