OAuth is a specific protocol for granting authorization to an application.

SSO (Single-Sign On) is the broader concept of signing into a single identity provider (like Google or Microsoft) and then using that authentication to sign into 3rd party applications. SSO may use different protocols to authorize the user in a 3rd-party app, including OAuth, SAML, OIDC.

For some organizations, SSO extends deeper than verifying the user to a third-party app. It may also control which applications each user can sign into and even what access level they should have within each 3rd-party app.