O5

O5 or O-5 may refer to:


 * The "Outreach 5", emerging economies in the Heiligendamm Process: China, Mexico, India, Brazil and South Africa
 * A symbol for the anti-Nazi Austrian Resistance, where the 5 stands for E and OE is the abbreviation of Österreich
 * USS O-5 (SS-66), a 1917 O-class submarine
 * HNLMS O 5, a 1913 O 2 class submarine of the Royal Netherlands Navy
 * SM UC-99, a U-boat renamed O-5 for Japanese service from 1920 to 1921
 * Douglas World Cruiser, also designated as Douglas O-5
 * De Havilland Canada Dash 7, also designated as De Havilland Canada O-5
 * LNER sub-class O4/5, a subclass of the LNER Class O4, locomotive class
 * O-5, the pay grade for the following officer ranks in the U.S. uniformed services:
 * Lieutenant colonel in the Army, Marine Corps, Air Force, and Space Force
 * Commander in the Navy, Coast Guard, Public Health Service Commissioned Corps, and NOAA Commissioned Officer Corps
 * Otoyol 5, a toll motorway in Turkey
 * The time-displaced X-Men, also called "The Original 5"
 * The senior “overseer” position in the SCP Foundation