OpenSAML is a set of open source C++ & Java libraries meant to support developers working with the SAML protocol. The current version supports SAML 1.0, SAML1.1, and SAML2.0 protocols. In other words, OpenSAML is a library for handling the low level SAMLx.x protocol implementations.
SAML Toolkits are mostly free and open-source, designed to shorten the development time and reduce the complexity to implement SAML2.0 enabled web-apps.
SAML Java Toolkits from Okta uses OpenSAML Java libraries to implement minimal features of SAML2.0 protocol for vendors to quickly deploy SAML Service Provider (SP) server and protect their web-apps. Otherwise, vendor has to employ developers who has to know and understand SAML2.0 specification and then use OpenSAML or other similar libraries to implement SAML2.0 protocol to protect their web-apps.