universal donor

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who has group O blood and is therefore able to serve as a donor to a person of any other blood group in the ABO system.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Med.) a person belonging to the O blood group (of the ABO classification), who can safely donate blood to anyone.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person with blood type o, which lacks antigens, and is therefore compatible with all other blood types, allowing such person to donate blood to any other person.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person whose type O Rh-negative blood may be safely transfused into persons with other blood types