O class or Class O may refer to:

Locomotives

• 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

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

This disambiguation page lists articles associated with the title O class.
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