It was instituted 6 September(?) 1886 by Queen Victoria, the Royal Warrant being published on 9 November. It usually goes Majors and above, but is sometimes awards to especially valorous junior offciers. 8,981 DSOs were awarded during World War I, each award being announced in the London Gazette.
The ribbon is red with blue edges, and the medal proper is a scalloped cross shape, white and gold with a wreath and crown in the center. Recipients are entitled to use "DSO" or "D.S.O." after their names.
See also: Army/Navy/Air Force Distinguished Service Orders of Vietnam