Fhei Tomo, we differ beween authentication and authorization. The login with okta is only the authentication part, so it has nothing to do with the project and if the user has permission to it.
FYou can configure the default settings wich will be taken for all organizations.
FAnd then you can overwrite them in the organization settings
Fso in your case you can just add the okta login in organization B settings,
FYes, thats right