With OAuth 2.0 (IETF RFC 6749) you sign in on the other service's own page, such as Google, Microsoft or Intuit, and approve the access the requesting application asks for. The service then issues that application an access token limited to what you approved. Your password is never given to the application.
Access can be withdrawn at any time by revoking the token, from the application or from the account's security settings, without changing your password.
MetaWurks connects tools this way: each person signs in on the app's own screen, MetaWurks never stores passwords, and disconnecting a tool revokes its access token.
General information, not legal or tax advice.