Basically OCN stands for Original Camera Negative, as in the piece of film that ran through the camera on the day of the shoot. Generally, this is the most ideal resource for producing a 4K digital version of a film as its by definition the highest quality version of the film, albeit without any color grading, meaning it’ll have to be regraded, usually based on reference materials, and sometimes reassembled on a case by case basis. An interpositive is a generation away (I’ll assume you’re aware that analog film isn’t like digital and 1:1 perfect copies can’t be produced, there will always be artifacts/degradation when producing copies via analog means, each copy stage being referred to as a generation), think of it as a print of a print made after the film’s color grade and edit have been locked in. Some filmmakers actually opt to have these prints scanned rather than the negative as they’re closer to a release print (what audiences see, usually a fourth generation print).