Officer Candidate School (OCS) (USMC)

USMC: As a Marine, all Officer Candidates are required to attend OCS regardless of commissioning source with the sole exceptions of graduates of the United States Naval Academy. The mission of USMC OCS is to "Screen Train and Evaluate" with all the emphasis on the Screening and Evaluating. It is difficult.

Other Services' OCS have different requirements and standards, I'll leave thsoe for them to comment on.

USMC: The focus is on screening and evaluating, a candidate learns only the most elementary basics to determine whether he or she possesses the basic requirements to serve as a Marine Officer.

USMC: Get in amazing shape, learn all the land navigation and small unit formations you can before arriving. Talk to as many recent graduates as possible. Embrace the uncertainty. Refuse to quit.