Is there guidance for what defines an 'Officer' for a Financial Institution? All I am aware of is the Reg. O definition of Executive Officer.

Try this: An employee of a company who shares legal liability for his/her company's actions, can sign or commit the institution to perform certain transactions or deeds, etc.

You might also try 12 CFR 561.35 (a Savings Association Reg), which states:
The term “Officer” means the president, any vice-president (but not an assistant vice-president, second vice-president, or other vice president having authority similar to an assistant or second vice-president), the secretary, the treasurer, the comptroller, and any other person performing similar functions with respect to any organization whether incorporated or unincorporated. The term “officer” also includes the chairman of the board of directors if the chairman is authorized by the charter or by-laws of the organization to participate in its operating management or if the chairman in fact participates in such management.

The institution can define that for itself, but basically, it's going to be someone elected by the Board of Directors as an officer (for insurance and contract purposes), who can act on behalf of the institution legally. The Board sets those parameters.

The definition must be set, though, because of D & O liability insurance reasons if nothing else!

Another place to check is within the by-laws of the organization.