DCO is a direct commission officer. Such commissioning of civilians was widely used in World War II to bring industrial management expertise, scientific or medical skills quickly into the U.S. armed forces.
Some people are given direct commissions into the military as officers due to their specific experience or special skill. An example of this today would be someone with a legal, dental, or medical degree.
Occasionally, some industry experts in areas such as special engineering, research administration or management would be commissioned straight into uniform.
However, today direct commission officers rarely have authority outside of their specific speciality in that command structure.
During World War II, Lt. General William Knudsen with the highest-ranking such commission, is possibly the most famous example.
Another famous example is Lt. Col. Jackie Cockran, commander of the WASPs - Womens Air Force Service Pilots.
Source: US Army Museum Ft. Belvoir