n. pl. o's or
O's also
os or
Os 1. The 15th letter of the modern English alphabet.
2. Any of the speech sounds represented by the letter o.
3. The 15th in a series.
4. Something shaped like the letter O.
5. O One of the four major blood groups in the ABO system. Individuals with this blood group have neither A nor B antigens on the surface of their red blood cells, and have both anti-A and anti-B antibodies in their blood serum.
6. A zero.