The Distinguished Service Order (DSO) is a military decoration of the United Kingdom, and formerly of other Commonwealth countries, awarded for meritorious or distinguished service by officers of the armed forces during wartime, typically in actual combat.

The DSO was instituted on September 6 by Queen Victoria in a Royal Warrant published on November 9. Typically, awarded to officers ranked Major (or its equivalent) or higher, the honor is sometimes awarded to especially valorous junior officers. 8,981 DSOs were awarded during World War I, each award being announced in the London Gazette.
The order was established for rewarding individual instances of meritorious or distinguished service in war. Though a military order, until recently for officers only, and normally given for service under fire or under conditions equivalent to service in actual combat with the enemy, it was awarded between 1914 and 1916 under circumstances which could not be regarded as under fire (often to staff officers, which caused resentment among front-line officers). After January 1, 1917, commanders in the field were instructed to recommend this award only for those serving under fire. Prior to 1943, the order could be given only to someone Mentioned in Despatches. The order is generally given to officers in command, above the rank of Captain, junior ranks were usually awarded the Distinguished Conduct medal (for soldiers and NCOs) or the Military Cross for officers below the rank of Captain. A number of more junior officers were awarded the DSO, and this was often regarded as an acknowledgement that the officer had only just missed out on the award of the Victoria Cross.
During WW2 the DSO was awarded to 870 RAF officers, with 62 receiving a first bar, 8 a second bar, and 2 a third bar.
Since 1993, its award has been restricted solely for distinguished service (i.e. leadership and command by any rank), with the Conspicuous Gallantry Cross being introduced as the second highest award for gallantry.
Recipients of the order are officially known as Companions of the Distinguished Service Order. They are entitled to use the post-nominal letters "DSO". A bar is added to the ribbon for holders of the DSO who receive a second award.
The medal signifying its award is a gold cross, enamelled white and edged in gold. In the centre, within a wreath of laurel, enamelled green, is the Imperial Crown in gold upon a red enamelled ground. On the reverse is the Royal Cypher in gold upon a red enamelled ground, within a wreath of laurel, enamelled green. A ring at the top of the medal attaches to a ring at the bottom of a gold "suspension" bar, ornamented with laurel. At the top of the ribbon is a second gold bar ornamented with laurel. The red ribbon is 1.125 inches wide with narrow blue edges. The medals are issued unnamed but some recipients have had their names engraved on the reverse of the suspension bar.
The bar for a second award is plain gold with an Imperial Crown in the centre. The back of the bar is engraved with the year of the award. A rosette is worn on the ribbon in undress to signify the award of a bar.