There are different Usages of the letter O.
1. the fifteenth letter of the English alphabet
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
This letter denotes the next after N in a set of items, categories, sizes, etc. e.g. It also denotes the fifteenth of fifteen or more hypothetical people or things.
O = zero specially when spoken. O = Oh as exclamation. O' = Of with apostrophe. In some scientific notations especially structural chemistry o = ortho as in o-xylene.
In Chemistry O = Oxygen the eighth element of the periodic table.
The most Common Noun which is used to learn this letter at the Nursery level is Orange; O for Orange.