To enable OAuth, you will need to use an administrator account to navigate to Gear > Site Configuration > Authentication and choose either “Google OAuth Authentication” or “Username and Password or Google OAuth Authentication” from the “Authentication Mode” dropdown menu:
When you click the “Sign-in with Google” button to login, OAuth will try to match your google email address with a FogBugz account. If your email address doesn’t match an account in FogBugz or if there are multiple FogBugz user accounts with the same email address, authentication will not succeed. We recommend switching to Username/Password and OAuth temporarily before switching to OAuth-only just in case there’s an issue with signing in with Google.
If you have set FogBugz to allow users with email address at a particular domain to add themselves and a new user has a Google Apps email address with a valid domain, they can automatically sign up for an account. OAuth can also be used in combination with two-factor authentication.