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
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