M. O.

Printed state rosters and regimental histories squeeze each soldier's service into one line, and m. o. (mustered out) is one of the most common short forms. It usually comes with a date, as in m. o. July 1, '65.

Do not confuse it with mus., which some rosters use for mustered into service, the start of service. Each roster prints its own key, so check the abbreviation list at the front of the book.