The United States Marine Corps Officer Candidates School (OCS) is a training regiment designed to screen and evaluate potential Marine Corps officers at Marine Corps Base Quantico in Virginia. Those who successfully complete the period of instruction are commissioned as second lieutenants.

Most Marine Corps officers earn their commissions through OCS, though others are midshipmen from the United States Naval Academy, limited duty officers and warrant officers, and inter-service transfers. This differs from the other United States military services, who commission the majority of their officers through the Reserve Officers’ Training Corps (ROTC) programs at various colleges.

Officer candidates go through a 10-week Officer Candidates Course (OCC) or Platoon Leaders Class (PLC), or two 6-week PLC courses over separate summers, designed primarily to screen and evaluate candidates' physical and mental capabilities to lead Marines by placing them in leadership positions and physically and mentally demanding environments. The second 6-week course only happens after a candidate's junior year of college. The OCC 10-week course is available to those seeking a commission who meet the requirements to become an officer to include already possessing a bachelor's degree. Students are evaluated during two- to three-day garrison command billets at the company, platoon, and squad level, and fire-team level tactical billets during field exercises and situational leadership events.

Before World War I, Marine officers came primarily from the Naval Academy or were commissioned from the enlisted ranks. But as the Marine Corps expanded, it needed its own training pipeline for officers. OCS traces its roots to the "School of Application," established in 1891 in Washington, D.C. With the expansion of the Marine Corps for World War I, all instructional efforts were consolidated—first at Marine Corps Station, Philadelphia, then in 1940 at Marine Corps Base Quantico. PLC Juniors was at Camp Upshur through 1987 but was subsequently consolidated with PLC Seniors at Brown Field, where they remain today.

There are several ways to gain entry to Officer Candidates School:

Officer Candidates must pass a series of tests before being admitted into the Officer Candidate School. An Officer Selection Officer (OSO), usually a Captain, meets with a prospective Officer Candidate. Upon completing a satisfactory interview, the OSO then makes the decision to move the prospective candidate onto the next step. The prospective candidate then must complete a short essay about why they want to be a Marine Officer, provide identification (usually a birth certificate and Social Security card), pass a background check, provide five letters of recommendation, and complete a physical medical exam.

After successful completion of these steps, the OSO may then conduct the Officer Candidate through a Physical Fitness Test. Upon reaching a score on the test that the OSO deems to be acceptable (a minimum first class score is necessary), the Officer Candidate then signs the contract (including the contract to attend the course, the fraternization policy acknowledgement, and the Marine Corps drug policy acknowledgement). Candidates choose to enter the program as either a ground, air, cyber, or law contract.

All of this information is sent to a review board, which will vote to decide if the Candidate should be accepted to Officer Candidate School. These review boards generally convene only once a month. After receiving a majority vote of acceptance from the review board, the Officer Candidate is officially accepted into the Officer Candidate Program and scheduled for a class.