A good OC comes to a new team and all of a sudden they find talent in the existing roster. Maybe they recognize hidden existing talent. I suspect they do a better job of coaching up and elevating talent.
BSU is a great example. For years they've taken scrubs and turned them into a string of NFL running backs. These are kids who would be 2nd string at BYU and ride the pine at a P5, and BSU coaches turn them into ballers.
You should expect your OC to produce playmakers. Turning regular kids into playmakers is a huge part of the role.