When you are looking at grammar there are differences between letters and numbers. "0" is a numerical value that is used in math, statistics and computer programing that indicates a null value or no numerical value. An example of this is 2 X 0 = 0. When you multiply the number 2 by 0 you are left with the value of 0.
When you are looking at "o" you are looking at a letter within the English alphabet. The letter "o" does not have any meaning by itself. The letter "o" can be used as a variable within math problems but does not hold numerical value itself.