I believe so, by 215.3(a)(7)"An extension of credit is making or renewing of any loan...and includes: Any other similar transaction as a result of which a person becomes obligated to pay money (or its equivalent) to a bank, whether the obligation arises directly or indirectly, or because of an endorsement on an obligation or otherwise, or by any means whatsoever." John is obligated to pay Mark's loan in the event that Mark defaults. I believe there is wording in the promissory notes to the effect that if Mark doesn't pay, the bank can go straight to John for collection, bypassing Mark. So it should be included in John's Reg O total as a reportable loan.