O class

O class or Class O may refer to:

Rail transport

 * Highland Railway O Class, a British class of 2-4-0, later 4-4-0T, steam locomotives built in the late 1870s
 * NER Class O, a British 0-4-4T steam locomotive class
 * NZR O class, a locomotive class of New Zealand steam engines built in the United States
 * Russian locomotive class O, an early type of Russian steam locomotive
 * SER O class, a British 0-6-0 steam locomotive class
 * WAGR O class, Australian 2-8-0 steam locomotive class built in the late 19th century
 * O-class Melbourne tram
 * O-class Sydney tram

Stars

 * Class O stars, the rarest of all main sequence stars

Vessels

 * O and P-class destroyer, the first ships built under the United Kingdom's War Emergency Programme destroyers
 * O-class battlecruiser, a class of warships planned for the German Navy just prior to the Second World War


 * O-class submarine (disambiguation)
 * Class O, several early submarine classes of the Royal Netherlands Navy
 * Oberon-class submarine
 * Odin-class submarine
 * United States O-class submarine