I have heard that this first release of 12c doesn't have an upgrade path. I dare say its because of the postgres DB -> oracle DB which maybe the issue.
But I have heard on the grapevine that the 12.1 release should have the upgrade path. But I cannot say this for certain. As I'm just a user myself.
The upgradable media for Ops Center 12c is not available at this time. Should you need details regarding time estimates as to when that media will be available, please reach out to your local Oracle Systems account team. Oracle can better share information about unreleased software in direct one on one conversations. On public aliases such as this, we would only be able to say soon.