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

Solution 1:

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

Solution 2:

Actually I do believe that the rule is:

• -oes for most words ending in -o: potatoes, tomatoes
• -os for any words related (usually) to music (i.e. loanwords, from Italian). These would include: pianos, crescendos, radios, concertos, sopranos, pimentos (loanword), etc.

Solution 3:

If the noun ends with vowel + o, just add -s: radios, zoos.

If it’s a music word, it’s probably Italian, so just add -s: pianos, concertos.

Otherwise, look it up in a dictionary. There is no rule of thumb that correctly covers all words. Many nouns ending with -o can take either -es or -s but one or the other is preferred.

(This answer is informed by a recent attempt of mine to boil down CGEL’s spelling rules for plurals and suffixes to something my kids could memorize. CGEL is the Cambridge Grammar of the English Langauge. I was not all that successful.)