O
- OCONUS – Outside of CONtinental United States, as opposed to CONUS.
- O-course – obstacle course.
- OCS – Officer Candidate School, recruit training for officers.
- O-dark thirty – very early hours before dawn. See also military time. The custom of saying "oh" instead of zero has diminished, but remains in this expression.
- office hours – administrative ceremony where legal, disciplinary, and other matters (such as praise, special requests, etc.) are attended, designed to dramatize praise and admonition, in a dignified, disciplined manner, out of the ordinary routine. Known as Captain's Mast afloat. An award given during a positive office hours or Mast is known as a Meritorious Mast, a negative office hours with punishment awarded is an example of non-judicial punishment.
- officers' club or officers' mess or O-Club – recreation facility for officers that often includes a bar, restaurant, game room, and objects of unit significance, such as a mascot or war trophy; similar to a gentlemen's club.
- officers' country – living spaces for officers aboard ship, or portion of post or station allocated for the exclusive use of officers.
- OFP – Own Fucking Program, not complying perfectly or synchronized with orders assigned to a group.
- OJT – On-the-Job Training, without a formal school or period of instruction.
- Oki – Okinawa.
- old Asia hand – person with more than one tour in Asia.
- Old Man – very informal nickname for the commanding officer, considered an inappropriate term of endearment for use by a junior, thus used in reference but never in address.
- OMPF – Official Military Personnel File, a record of all awards, punishments, training, and other records compiled by Headquarters Marine Corps.
- oorah or ooh rah or Urah – spirited cry used since the mid-20th century, comparable to Hooah used in the Army or Hooyah by Navy SEALs; most commonly used to respond to a verbal greeting or as an expression of enthusiasm. The origin is often disputed.
- OP – Observation Post, a position used for reconnaissance; also, the post newspaper of Marine Corps Air Ground Combat Center Twentynine Palms.
- operational tempo or optempo – the pace or speed of operations for a unit or individual.
- OPSEC – Operation(s/al) Security, counterintelligence efforts to keep generally unclassified but sensitive information (such as troop movements and deployments) from enemies.
- OPTEMPO – OPerational TEMPO, or the pace of operations and activities for a given unit.
- OQR – Officer Qualification Record, a service record for officers, much like an enlisted Marine's SRB.
- OOB – Out Of Bounds, or straying into an area restricted from use by normal traffic, prohibited to Marines, or too far from base for a given liberty period.
- OOD – Officer Of the Deck, or the senior Marine responsible for the patrol and security of a unit's garrison working spaces and sleeping quarters after working hours, usually responsible for subordinate sentries and acts as a guard commander. See also duty & firewatch
- Oscar Mike – On the Move, the names of the two NATO phonetic alphabet letters O and M which stand for the phrase. Used on the radio and in shorthand to each other. See also NATO phonetic alphabet
- OTV – Outer Tactical Vest, militarized version of Interceptor body armor, a common type of ballistic vest; being replaced by the MTV.
- outside – civilian life after discharge. See also real world.
- overhead – ceiling.
- over the hill – excessively old; or to desert.
Read more about this topic: List Of United States Marine Corps Acronyms And Expressions