Nouns ending in o preceded by a vowel form their plurals by adding -s to the singular: cameo, cameos; duo, duos; studio, studios; zoo, zoos.
Most nouns ending in o preceded by a consonant also usually add -s to form the plural: alto, altos; casino, casinos; ego, egos; Latino, Latinos; memo, memos; neutrino, neutrinos; poncho, ponchos; silo, silos. However, some nouns ending in o preceded by a consonant add -es: echo, echoes; hero, heroes; jingo, jingoes; no, noes; potato, potatoes; tomato, tomatoes. Some nouns ending in o preceded by a consonant have two plural forms (the preferred form is given first): buffaloes or buffalos; cargoes or cargos; desperadoes or desperados; halos or haloes; mosquitoes or mosquitos; zeros or zeroes. (The American Heritage Book of English Usage)