grammatical number - Is there a good rule of thumb for plurals of words ending in "o"?

The following words and their plurals seem to be somewhat inconsistent:

• combo / combos


• concerto / concertos


• grotto / grottos / grottoes (?)


• hero / heros (?) / heroes


• potato / potatos (?) / potatoes


• tornado / tornados / tornadoes


• tomato / tomatos / tomatoes


• volcano / volcanos / volcanoes

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.)

Answer

This 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