O (3 letters) The two crossword solutions for the term 'blood types, abbr.' are ABS and O. ABS stands for ABO Blood Group System, which is a classification system for human blood based on the presence or absence of certain antigens on the surface of red blood cells. O stands for the blood type O, which is the most common blood type in humans and is also known as the universal donor type because it can be donated to any other blood type.