The Highland Railway O Class locomotives were built as 2-4-0T tank engines, but were soon rebuilt as 4-4-0Ts.
This is a list of submarines of the Netherlands navy.
The NER Class O (LNER Class G5) was a class of 0-4-4T steam locomotives of the North Eastern Railway, designed by the company's Chief Engineer, Wilson Worsdell.
The O class consisted of six steam locomotives that operated on New Zealand's national rail network.
The Russian steam locomotive class O (from Основной) was an early type of Russian steam locomotives.
The South Eastern Railway (SER) O Class (some of which were later rebuilt, becoming the O1 Class) was a class of 0-6-0 steam locomotive designed for freight work, and were the main freight engines of the SER, and later the South Eastern and Chatham Railway (SECR) for a number of years.
The United States Navy's sixteen O-class submarines were created out of the lessons learned from the L class.
The WAGR O Class was a 2-8-0 steam locomotive introduced by the Western Australian Government Railways (WAGR) in 1896.