O1

noun (plural Os or O's /ōz/)

• 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) zero (in a sequence of numerals, especially when spoken).

• 3a shape like that of a capital O; a circle.