At OCS, the Sergeant Instructors run the day to day aspects of OCS. For new candidates, they are the Marine Corps in flesh and blood. They demand more from you than anyone else and they demonstrate by their own example. Prior to becoming a Sergeant Instructors at OCS, each Marine must complete a 3-4 year assignment at one of the recruit depots. After this, they return to their primary MOS. During the academic year, most are the Assistant Marine Officer Instructors at NROTC units across the country.
Platoon Commander (Captain)
The Platoon Commander, a Captain, oversees training and teaches officer specific classes to the platoon. Additionally, they lead PTÂ and demonstrate the OCS motto, “Lead by example.” Often they exercise more of an administrative role than the enlisted staff.
Platoon Sergeant (Staff Sergeant or Gunnery Sergeant)
Much like a Senior Drill Instructor, the Platoon Sergeant is the father figure of the platoon. The Platoon Sergeant will focus on making corrections to billet holders rather than correcting “general population” candidates. They are more concerned with the big picture and guiding their Sergeant Instructors. Additionally, they provide mentorship and guidance to candidates in leadership billets. This Sergeant Instructor is typically the most experienced. They know how OCS really works and make sure everything runs smoothly.
Heavy (Staff Sergeant or Gunnery Sergeant)
The Heavy is in charge of upkeep and procedures in the squad bay as well as teaching candidates most of their drill. They will become familiar with your deficiencies and will watch for opportunities to fix these via essays and chits. If your platoon has four Sergeant Instructors, two will act as the heavy.
Kill Hat (Staff Sergeant or Gunnery Sergeant)
The Kill Hat exists to create friction, induce stress and enforce discipline. Unlike the first three Sergeant Instructors, the kill hat is not going to provide a lot of mentorship or tell you how to lead. They are an opportunist who will use every chance to correct any deficiency.
The role of Sergeant Instructors
At first, you will fear the Sergeant Instructors and they will be the common enemy of the platoon. Then, you will realize that no one has put so much effort into making you better and demanded more from you personally than they have. All of the “games” have a purpose. They will feel stupid when it’s happening, but 
weeks later you will notice a change in yourself and the platoon. The best advice I can give you is, “Trust the process.” Contrary to popular belief, the staff is not there to just make your life miserable. They are there to produce a Marine and they take that job very seriously.
You will develop a deep level of respect and admiration for your Sergeant Instructors. For many, making them proud will become more important than earning your EGA. You will talk to Marines who went to boot camp or OCS 30 years ago and they will remember the names of their Drill Instructors/Sergeant Instructors.