When it comes to donated blood, type O is special. It can be given to anyone, regardless of their blood type. By contrast, type A can only go to A or AB patients, and B can only go to B or AB patients. Additionally, type O patients can only receive O. Thanks to new research, however, it may soon be possible to give anyone whatever type of blood happens to be available, with no ill effects.
The reason that type O is so transfusion-friendly is that it contains no A or B antigens – these will provoke a severe immune response in bodies where they aren’t already present.