1the fifteenth letter of the alphabet. denoting the next after N in a set of items, categories, etc.. a human blood type (in the ABO system) lacking both the A and B antigens. In blood transfusion, a person with blood of this group is a potential universal donor.
2 (also oh) nought or zero (in a sequence of numerals, especially when spoken): two seven o seven point six
3a shape like that of a capital O; a circle: he made an O with his mouth
atomic number 8