ISCC opens up the challenge of multiple registrations of the same piece of content by multiple parties. How is trust established when anyone can claim they wrote the Beatles back-catologue?
OCCP is a method to register certification entities. These aren't necessarily the same entity as the content author, or owner - but they are permissioned with registering metadata around a specific ISCC code; for instance a record label, a production company, a photography club or website.
OCCP doesn't confer any information on an ISCC other than it was registerd by a certified entity, which at a minimum is a legal entity, as recognised with a Legal Entity Identifier ([[LEI]]), which can be generated for a fee.