An officer candidate school (OCS) is a military school which trains civilians and enlisted personnel in order for them to gain a commission as officers in the armed forces of a country. How OCS is run differs between countries and services. Typically, officer candidates have already attained post-secondary education, and sometimes a bachelor's degree, and undergo a short duration of training which focuses primarily on military skills and leadership. This is in contrast with a military academy which includes academic instruction leading to a bachelor's degree.

A non-commissioned officer (NCO) is a military officer who does not hold a commission. Non-commissioned officers usually earn their position of authority by promotion through the enlisted ranks. In contrast, commissioned officers usually enter directly from a military academy, officer training corps (OTC) or reserve officer training corps (ROTC), or officer candidate school (OCS) or officer training school (OTS), after receiving a post-secondary degree.