People with O- blood are called "universal donors" because any patient can receive O- red blood cells. Hospitals use O- red blood cells to provide emergency care for patients needing immediate transfusions. In addition, hospitals maintain a supply of O- blood for patients with an O- blood type, who can only receive O- red blood cells.
On the other end of the spectrum, very few people can receive AB+ red blood cells. However, people with AB+ blood can receive red blood cell transfusions of any blood type, and are known as "universal recipients." AB blood is special for another reason; plasma donations from people with AB blood can be safely transfused to anyone, so AB donors are "universal plasma donors."
To learn more about the distribution of blood types in America, click here.