Google Books Ngram Viewer — banjos vs banjoes — English 
Is there a common source for -s versus -es? With regards to words such as "heros", is it likely to see more words start dropping -es in favor of -s?
(If you have any other good examples of strange plurals from words ending in "o" feel free to edit them into the question.)
67.1k 10 10 gold badges 155 155 silver badges 226 226 bronze badges asked Apr 29, 2011 at 16:25 35.8k 32 32 gold badges 124 124 silver badges 265 265 bronze badges I don't think you can include "combo" in that list. It's slang for "combination". Commented Apr 29, 2011 at 16:28@Al: "Combo" has its own entry in my dictionary along with a rule for pluralizing it. It being slang for "combination" is likely why it gets -s instead of -es which, presumably, would be part of the answer to this question. But "combo" is a word and it is pluralized and it does end in "o". Therefore, I consider it relevant to this question.
Commented Apr 29, 2011 at 16:35 I think Al has it right: combo is slang and that's why it's irregular. All the others are -oes . Commented Apr 29, 2011 at 16:37@Al, @Mal: If you think that is the answer than post it as an answer. :) Simple but correct answers are still correct. But I am still curious about potatos.
Commented Apr 29, 2011 at 16:40 Never seen it spelled "potatos". Commented Apr 29, 2011 at 16:51This is what The Cambridge Grammar of the English Language (p. 1586, The alternation between ·s and ·es) has to say about it:
With bases ending in o, where o does not follow a consonant symbol (i.e. where it is preceded by a vowel or is part of the composite vowel symbol oo), the plural takes ·s:
bamboos, cameos, embryos, folios, kangaroos, patios, radios, studios, zoos
Where o does follow a consonant, the plural has to be specified for the lexeme concerned. There are three classses:
i. ·es only: echo ~ echoes. Also domino, embargo, hero, mango, negro, potato, tomato, torpedo, veto
ii. ·s or ·es: motto ~ mottos/mottoes. Also, archipelago, banjo, buffalo, cargo, dado, dodo, grotto, halo, innuendo, manifesto, mulatto, proviso, tornado, volcano
iii. ·s only: bistro ~ bistros. Also calypso, do, dynamo, beano; clippings such as demo, kilo, memo, photo; nouns of Italian origin: cello, concerto, contralto, libretto, maestro, piano, quarto, solo, soprano, virtuoso; and names of ethnic groups: Chicano, Eskimo, Filipino, Texano.
Cargo and volcano are marginal members of class [ii]: they usually take ·es, but the forms cargos and volcanos are sometimes found.
As an additional rule of thumb, almost of all the exceptions for the consonant + o + s = es rule seem to involve plural nouns where there is no homophonous verb, as per CGEL, p. 1580.
For class [ii], it might be useful to use Google Books Ngram Viewer to assess the ·s/·es distribution, and to go with the ending that seems to be preferred in literary sources. For instance,
Google Books Ngram Viewer — mottos vs mottoes — English Google Books Ngram Viewer — banjos vs banjoes — English
