While often based on your unit's BOS, a SF Marine's Marine Occupational Specialty (MOS) does not have to be the same as the unit's BOS, it may however limit your choices in uniform and insignias.  Your MOS is the fictional specialization your Marine has within your chosen branch.  Below are sample MOS's within the various BOSs within the SFMC.

• Aerospace:  Examples of MOSs in this branch include pilot specialties for all the assorted crafts, as well as jobs like Aerospace Intercept Officer and EW Specialist. Common missions in Aerospace include ground support, aerospace superiority, interdiction, and interception. Because of that last type of mission, Aerospace pilots are considered either very brave or insane. Of course, they feel the same way about the Infantry.
• Armor:  MOSs in the Branch include Armored Fighting Vehicle Pilot/ Vehicle Commander, Artillery Spotter, Infantry Fighting Vehicle Systems Operator, and many more.
• Combat Engineers:  MOS examples in the Branch include Heavy Construction Equipment Operator, Field Scientist (Geology), Combat Ecologist, Sapper, Diver, and CBR Specialist. 
• Infantry:  Examples of MOSs in this branch include the basic Infantryman, as well as Light and Heavy Weapons specialists, Scout/Snipers, and a variety of unique Powered Infantry specialties.
• Maritime Operations:  MOS examples include Signalman, Ocean System's Technician, Master at Arms, Gunner's Mate, Turbine Technician,
  and Hull/Repair Technician.
• Medical:  Examples of MOSs within this branch include Combat Medics, Nurses, Flight Surgeons, and Xenophysiologists. It is important to note that SFMC medical personnel often function where there is no advanced support by STARFLEET medical assets like a ship's medical section. They must face the dual challenge of keeping the wounded alive with a minimum amount of equipment and staying alive themselves (as the enemy often disregards the noncombatant status of medical personnel).
• Mech:  MOSs includes pilot specialties for the wide range of Mech vehicles, as well as jobs for systems operators and support personnel, which naturally come with fielding such a technologically complex combat force.
• Special Operations:  Examples of MOSs in this branch include Weapons Sergeant, Team Medic, Interpreter/Linguist, and Data Warfare Specialist.
• Support:  Examples of MOSs within this branch include Quartermasters, Intelligence Analysts, Computer Technicians, Military Police, Musicians, and a host of other MOS descriptions that fall outside of the direct- combat concept. The Support Branch contains nearly 60 distinct organizational components which employ millions of Marines and civilians.

More information on each of these MOSs can be found in the Brigade Library under each Occupational manual.