The plurals of words ending in -o are formed by either adding -s or by adding -es. The plurals of many words can be formed either way. To determine whether a particular word ends in -s or -es (or if the word can be spelled either way), check your the list below and if you have to check your dictionary. There are two helpful rules:

a. All words that end in a vowel plus -o (-ao, -eo, -io, -oo, -uo) have plurals that end in just -s:

• stereo--stereos
• studio--studios
• duo--duos

b. All musical terms ending in -o have plurals ending in just -s.

• piano--pianos
• cello--cellos (I have been told that the plural for cello is actually, celli)
• solo--solos

c. Plural forms of words ending in -o:

-os	-oes	-os or -oes
albinos armadillos autos bravos broncos cantos casinos combos gazebos infernos kimonos logos maraschinos ponchos sombreros tacos torsos tobaccos typos	echoes embargoes heroes Negroes potatoes tomatoes torpedoes vetoes	avocados/oes buffaloes/os cargoes/os desperadoes/os dodoes/os dominoes/os ghettos/oes grottoes/os hoboes/os innuendoes/os
Purdue University Writing Lab 2000

This has to have a Dan Quayle link to it. Of course, it's a pity that Dan wasn't linked to this before he corrected that poor schoolboy.

The above makes sense if you happen to be discussing English. The following can be said about my mother tongue, Dutch:

Words on -o generally take 's for a plural: auto's, piano's, dodo's, libero's, foto's, gekko's, hobo's. The only exception I can think of is protozoën, which, incidentally, also takes its original Greek plural, protozoa.