‘OkTA for SSO’ is I assume just standard Okta? I couldn't find any reference to this.

Did you ask this on stackoverflow? I answered there.

I assume:

External client users authenticates with Okta

Your client users authenticate with B2C

You want the external client users to access your application

In which case the SO answer is the way to go.

Otherwise, please provide a diagram of what you want to achieve.