O1

O1 is a designation used in several unrelated fields to denote different concepts.

In biology, Vibrio cholerae O1 is the serogroup responsible for most epidemic cholera, with serotypes Inaba and Ogawa and biotypes Classical and El Tor.

In astronomy, O1 is the hottest spectral subclass of O-type stars, blue-white and extremely luminous. Subclasses such as O1V denote a main-sequence star; these subdivisions are less common than broader O-type classifications.

O-1 is the entry-level officer pay grade in many services. Army, Air Force, and Marine Corps use Second Lieutenant, while Navy and Coast Guard use Ensign; insignia typically features a single gold bar.

O1 is a high-carbon tool steel, oil-hardening widely used for tools and blades due to durability and ease of sharpening; heat treatment controls hardness and toughness.

In mathematics, O(1) denotes a constant bound in asymptotic analysis. O1 is not standard notation; conventional form is O(1).