When an Executive Officer co-signs a loan, is it permissible under Reg O to use the credit score of the Executive Officer to determine the interest rate on the loan or is it required to use the credit score of the person getting the loan to determine the interest rate?

How do you normally determine the interest rate? If you normally determine it on the borrower's score instead of the "co-signers" score then that is how you should do it for this loan. Otherwise I would say you're giving the EO preferential treatment because they are obligated on the loan.