The rule says that if a singular noun ends in consonant + "o" then the plural form will be consonant + "oes".
e.g. tomato => tomatoes.
Then, why this rule does not apply to piano?
Plural of nouns ending in -o:
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:
So you have:
solo solos, zero zeros, avocado avocados
Those which have a vowel before the final -o always just add -s:
studio studios, zoo zoos, embryo embryos.