O

Ⅰ.

O [1] (also o) ► NOUN (pl. Os or O's) 1) the fifteenth letter of the alphabet. 2) (also oh) zero, especially when spoken). 3) a human blood type (in the ABO system) lacking both the A and B antigens.

Ⅱ.

O [2] ► SYMBOL ▪ the chemical element oxygen.

Ⅲ.

O [3] ► EXCLAMATION 1) archaic spelling of OH(Cf. ↑oh). 2) used before a name in the vocative.