Adding -s or -es to words ending in O.
You can either add an -s or -es to words ending in O or sometimes they're spelled either way.

a. If a word ends in a vowel-a,e,i,o,u + O then we sometimes add -s.
radio/radios, stereo/stereos, video/videos

b. If a words ends in consonant + O then we sometimes add -s or -es.
No rules for this - you just have to learn them or use a dictionary.
kilo/kilos, zero/zeros, piano/pianos, photo/photos
but hero/heroes, potato/potatoes, volcano/volcanoes, tomato/tomatoes.

For more information about plural spelling rules click here