Words ending in
-o form plural by adding '-s'. In some resources I have read some of these words needs '-es' to make plural (e.g. echo, buffalo). However it seems both form of all such words (-s or -es) are recorded in dictionaries. So is it correct to form plural form of all o-ending word by only adding '-s'?
Words ending in
There is some good guidance given by Oxford Dictionaries (OUP) here showing quite a number of examples.
The basic guidance is
Nouns ending in -o can add either -s or -es in the plural, and some can be spelled either way.
As a general rule, most nouns ending in -o add -s to make the plural:
Those which have a vowel before the final -o always just add -s:
a list of the most common nouns ending in -o that are always spelled with -es in the plural:
singular plural buffalo buffaloes domino dominoes echo echoes embargo embargoes hero heroes mosquito mosquitoes potato potatoes tomato tomatoes torpedo torpedoes veto vetoes
some of the common nouns ending in -o that can be spelled with either -s or -es in the plural:
singular plural banjo banjos or banjoes cargo cargos or cargoes flamingo flamingos or flamingoes fresco frescos or frescoes
So there are some that 'require' the -es although there isn't a hard and fast 'rule' that you can apply.
Consider zero and hero; indistinguishable in their form and yet zeros is the standard plural of zero while heroes is the standard plural of hero.
"Plurals of nouns". Oxford Dictionaries. Oxford University Press. http://www.oxforddictionaries.com/words/plurals-of-nouns (accessed September 21, 2014).
Acording to this excellent presentaion, there are only a few words that must be pluriezed using only -es. Here is a list of such words:
(feel free to add any missing word to the list)