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 South Australian Railways O class (first), 2-8-0 steam locomotives South Australian Railways O class (second), 4-4-0 steam locomotive Victorian Railways O class, 0-6-0 steam locomotives 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 , the first ships built under the United Kingdom's War Emergency Programme destroyers , 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
United States O-class submarine