Question
What's the difference between Google OAuth and Google SAML? Do I need the Professional plan for Google OAuth?
Answer
SAML and OAuth are standards and not proprietary to Google.
SAML - Security Assertion Markup Language - it is XML based syntax that asserts that your identity (John/Jane Doe at XYZ Company) has been authenticated and the identity attributes (Name, address, phone, title, specific roles, etc) are described in XML Document, digitally signed (optionally encrypted) and sent to Receiving Party (Google).
OAuth - OAuth is not technically for Authentication. OAuth is used to control authorization - what you can or cannot do within a Service.
Google sign-in is available on all plans.