A person with blood group O, whose blood may be given to anyone.
A person whose blood is of the type 0 and Rh negative, which can be used for blood transfusions in most other people.
Person with type O, Rh-negative blood; this blood can be used with minimal risk for transfusion to people with types O, A, AB, and B blood.
A person whose blood is of group O and is therefore usually compatible with most other blood types. In actual practice this compatibility rarely occurs because of the many factors besides the major blood antigens (A, B, AB) that determine compatibility.