OCS forms one part A of the Basic Officer Leadership Course (BOLC A) and is 12 weeks in duration, with branch specific training forming part B (BOLC B) and is 6-16 weeks in duration. Candidates may then receive further training (schooling in the US vernacular), for example Airborne, before arriving at their first unit.
All candidates for OCS must submit an application packet (of documents) to the Human Resources Command Officer Professional Management Division (Officer Accessions Branch) where an OCS selection board will be conducted. The OCS does not have influence over what is required, nor who is (or is not) selected. Further information can be found here.
Warrant officers and enlisted personnel on active duty and in the Army National Guard may apply for OCS, as well as those in the US Army Reserve not on active duty. Certain former commissioned officers may also apply (as a rule you can only attend OCS once), and civilians under the OCS Enlistment Option.
Information on in-processing documents required and a packing list can be found in Useful Publications (Section 5.1). The packing list was revised on 11 Marc 2018, with those failing to adhere to the packing list risking being removed from the course! 153554b96e