An Order Book Official (OBO) is an employee of the CBOE who is responsible for maintaining a fair and orderly market in the options assigned to them and for executing orders that have been left with them by member firms.
Applying "Order Book Official (OBO)" to Securities Exams:
Order Book Officials are direct employees of the CBOE and cannot trade for their own account. Their responsibility is to maintain a fair and orderly market for the listed options assigned to them by the CBOE and to execute orders left on the book. This differs from DMMs / specialists on the NYSE or other exchanges who are members of the exchange, and who must trade for their own account in addition to maintaining a fair and orderly market and executing orders that are left on the book. On your exam you will most likely be tested on the role of the order book official and what they can or cannot do.