addOAuthService
is really only meant for OAuth 1 services.
For OAuth 2 you'll need to manually do the POST call. Here is an example
var parameters = {
method : 'post',
payload : 'client_id='+CLIENT_ID+'&client_secret='+CLIENT_SECRET+'&grant_type=authorization_code&redirect_uri='+REDIRECT_URL+'&code=' + code
};
var response = UrlFetchApp.fetch(TOKEN_URL,parameters).getContentText();
You can see a more fully formed example here - https://github.com/entaq/GoogleAppsScript/blob/master/IO2013/YouTubeAnalytics/oauth2.gs#L22