Single Sign-On (SSO) is the user-facing outcome that OAuth/OIDC, applied consistently across several applications, makes possible: log in once with one identity provider, and move between multiple applications without authenticating again at each one. It's the same OIDC mechanics covered throughout this reference, just applied by several different client applications against one shared authorization server rather than one application against one provider.
Federated identity is the broader idea SSO is one instance of: trusting a third party's authentication decision instead of managing credentials yourself at all. An enterprise using an identity provider like Okta or Azure AD for every internal tool is running federated identity — each internal tool is a separate OAuth/OIDC client trusting one shared identity provider as its authorization server, exactly the "Authorization Server" role defined earlier in this reference, just deployed inside a company rather than as a public "Login with Google" button. The practical payoff is centralization: disabling one compromised account at the identity provider immediately locks that user out of every downstream application trusting it, rather than requiring separate action in a dozen separate systems each holding their own copy of that user's credentials.