OAuth is an authentication protocol that enables applications to request access to user's data.  The application sends the user’s browser to a predefined URL to start the authorization process and displays the consent screen showing what access is being requested. Once the user approves access, the server responds by redirecting the browser to the client’s app. 

While Google OAuth is required in order to use Google Contacts in your Contact Picker, Microsoft and Yahoo OAuth are not required. However, we do recommend that users create their own Microsoft and Yahoo OAuth in order to customize the OAuth screens to match your brand.