OFAC as you stated applies to all transaction, CIP applies to new customers establishing an ongoing relationship. For example if a new customer comes to your bank and applies for and is approved for a loan then you are required by regulation to perform CIP and by law not to conduct transactions with someone on the OFAC list. How do you do that? Well the only way I know of not to do business with someone on the OFAC list is to check them against the OFAC list.
Hope this clears things up.