Section 215.9 of Regulation O says, "Each executive officer of a member bank who becomes indebted to any other bank or banks in an aggregate amount greater than the amount specified for a category of credit in section 215.5(c) of this part, shall, within 10 days of the date the indebtedness reaches such a level, make a written report to the board of directors of the officer's bank."

My question is:
Does this section apply to loans to "corporations" controlled by an executive officer or just to executive officers individually or sole proprietorships
/partnerships? Would it make a difference if the executive officer guarantees the loans to his corporations?

I think the answer depends on whether the executive officer is liable on the extension of credit. The rule says the report must be made when the EO "becomes indebted" to the other bank or banks.
My reading of that would require the report if the EO guarantees or endorses the debt, or if the debt is to a true partnership. But the unguaranteed corporate loan would not trigger the requirement.