Suggested styles vary. The usual way to indicate the plural of a letter is by using the uppercase and adding a lowercase S, so it becomes Os. If the context is not clear, some guides allow using the apostrophe S (which should not be used to form other plurals), creating the style O's. There is a third style that uses the English practice of adding -es to some words ending in O, which creates the plural "oes" (this is not widely used).
Example: There are two Gs and two Os in the word Google.
Example: How many I's and how many O's are in the name Pinocchio?
The plural of football is footballs.
The plural of ivy is ivies.
The plural of ghetto is ghettos (without an e on the o).
Some plural words ending in "o" are photos, pianos, casinos, and zoos.
The word Jell-o is a proper noun, a trademark brand. You could have flavors of Jell-o but no plural.
For words that end in O, add either s or es.
Jack-o-Lanterns
The plural form of fungus is fungi.
o keefees
Lands ;o
The plural of "oasis" is oases.
Odessi