It is my understanding that the old "Regular vs Reserve" commission situation no longer exists. I believe everyone coming out of the Academies as well as the ROTC Schoalrships kid are all being commissioned as "Reserve" officers and compete at 0-4 for the "Regular" title. See below.
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Reserve vs. Regular Officers
Many folks are confused about the differences between a "Reserve Officer" and a "Regular Officer." Being a Reserve Officer does not mean the officer is serving in the Reserves. In the "old days," graduates of the service academies were commissioned as Regular Officers, while those commissioned under ROTC or Officer Candidate School (called Officer Training School in the Air Force), were commissioned as Reserve Officers, who then "competed" later during their careers to be appointed as Regular Officers.
Being a Regular officer means a better chance of being promoted, protects against RIFs (reduction in force), and allows an officer to serve longer.
These days, all officers (including Academy graduates) are initially commissioned as Reserve Officers, and compete among themselves for appointment to regular officer at the time they are considered for promotion to major (O-4). Major-selects who also win appointment to Regular status receive the advantages of being a Regular Officer. This means: