SAML is the de facto standard single sign-on protocol for enterprise-grade applications. If a SAAS app integrates directly with Okta or OneLogin, it probably does so with SAML.

There's a lot of functional overlap between SAML and OIDC/OAuth, but SAML is a very different (and idiosyncratic) protocol; the "what" is the same, but the "how" is very different.