In blood grouping, a person belonging to group O; i.e., one whose erythrocytes do not contain either agglutinogen A or B and are, therefore, not agglutinated by plasma containing either of the ordinary isoagglutinins, alpha or beta. ... (05 Mar 2000) ... 
a person whose blood is type O in the ABO blood group system; such blood is sometimes used in emergency transfusion. Transfusion of blood cells rather than whole blood is preferred.
Type: Term Definitions: 1. in blood grouping, a person belonging to group O; one whose erythrocytes do not contain either agglutinogen A or B and are, therefore, not agglutinated by plasma containing either of the ordinary isoagglutinins.
Universal donor: A person who is type O in the ABO blood group system and can donate blood to all recipients.
A person with blood type O who is able to donate blood to people with other blood types in emergency blood transfusions.
[n] - a person whose type O Rh-negative blood may be safely transfused into persons with other blood types
noun a person whose type O Rh-negative blood may be safely transfused into persons with other blood types
a person with blood of group O.