Berets of the United States Army
U.S. Army Special Forces wear the green beret because of their link to the British Commandos of World War II. The first Ranger unit, commonly known as Darby's Rangers, was formed in Northern Ireland during the summer of 1942. On completion of training at the Commando Training Depot at Achnacarry Castle in Scotland, those Rangers had the right to wear the British Commando green beret, but it was not part of the regulation uniform at the time and was disallowed by the U.S. Army. 
The 10th Special Forces Group (Airborne) had many veterans of World War II and Korea in its ranks when it was formed in 1952. Members of the 10th SFG began to unofficially wear a variety of berets while training, some favoring the red or maroon airborne beret, the black beret, or the green commando beret. In 1953, a beret whose design was based on that of the Canadian Army pattern, and which was rifle-green in color, was chosen for wear by Special Forces units.
David Oh
During the 2011 campaign, "The Philadelphia Daily News" reported that Oh's claim that he served in the U.S. Army Special Forces, commonly known as the Green Berets, was a stretching of the truth. Oh criticized the paper for publishing the tabloid-styled piece and for confusing military uniform and terminology of 1991 with that of 2011. He noted that he did indeed serve as a 2nd Lieutenant in the 20th Special Forces Group (Airborne), Army National Guard. He also stated that the reporter had official documentation that his MOS (military occupational specialty) was changed from 11-A, "Infantry Officer", to 180-A, "Special Forces Technician", and that he was assigned a green beret which he had full authority to wear during his time of service. Oh himself did not, however, complete Special Forces training. He defended his military service” but issued an apology not being aware of the uniform changes since 1993 and for any confusion that may have resulted. Several veterans and related groups stepped-forward to defend Oh, including the Green Beret officer who recruited him to join the U.S. Army Special Forces. Veterans criticized the "Daily News"' original story, including the front cover picture of Oh with a green beret photoshopped onto his head, as misleading. As a result, the United Veterans Council of Philadelphia endorsed Oh and the Vietnam Veterans Memorial Society made him an honorary member.