The power of OKTA lies in its thousands of pre-configured SSO relationships. It's important to remember that each one was set up manually with a unique client ID and secret, metadata file, etc.
Application integration is the most difficult part of most identity and access management projects. That is why outsourcing identity to a SaaS provider like OKTA or OneLogin is so appealing--they have done all the hard work for you.
There are no open source projects like OKTA because if you're using open source software, you will inevitably have to integrate and test each application you want configured for SSO.