O9

O9 or O-9 may refer to:


 * USS O-9 (SS-70), an O-class submarine of the United States Navy
 * O-9, the pay grade for the following senior officer ranks in the U.S. uniformed services:
 * Lieutenant General in the Army, Marine Corps, Air Force, and Space Force
 * Vice Admiral in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps
 * Douglas O-9, a variant of the Douglas O-2, a 1920s American observation aircraft
 * O9 (model railways)
 * Oregon-IX (OIX), an internet exchange; see List of Internet exchange points