The capital Greek letter (capital omnicron) is visually very similar to the uppercase Latin letter O. For that reason, refer to the usage of Capital O for how the symbol appears in math.
Symbol  Format  Data 

Ο  Unicode 

TeX 


SVG 
The Greek Alphabet is used throughout math to represent variables, constants, and coeffecients within math expressions, formulas, and equations.
The Greek letter ο (omicron).