If we have a possible OFAC hit can we tell the customer that we have to review the OFAC information before we can process the transaction?

There is nothing confidential about OFAC. However, I would be very careful about how this is communicated and I'm not sure that a teller is the appropriate resource to have this discussion with a client.

Pretty much all cases these end up being false positives but they take a little more time to make this determination. What is the best way to communicate to the customer that they need to wait for us to make this determination without mentioning OFAC?

I don't think you have to avoid mentioning OFAC. I just think the tellers need to guided on how to have the conversation. I would imagine you require some identification from the person conducting the transaction, regardless of a potential OFAC match. If the tellers are trained properly, I don't think that addressing a potential OFAC match would significantly impact the transaction processing time.