area / areas
boa / boas
cola / colas
hula / hulas
idea / ideas
pea / peas
sea / seas

ace / aces
bone / bones
canoe / canoes
cove / coves
chute / chutes
dime / dimes
file / files
foe / foes

gate / gates
hare / hares
hoe / hoes
line / lines
mite / mites
nose / noses
oboe / oboes
pole / poles
quire / quires
site / sites
tile / tiles
toe / toes
woe / woes
vine / vines

ski / skis

A few regular countable nouns ending in
o form plurals by adding -es:

echo / echoes
hero / heroes
potato / potatoes
tomato / tomatoes

Many regular countable nouns ending in
o (including numerous musical terms) form
their plurals by adding only -s:

albino / albinos
alto / altos
auto / autos
basso / bassos
cello / cellos
duo / duos
folio / folios
ghetto / ghettos
halo / halos
kangaroo / kangaroos
kilo / kilos
kudo / kudos
memo / memos
photo / photos
piccolo / piccolos
piano / pianos
radio / radios
solo / solos
soprano / sopranos
studio / studios
tatoo / tatoos
trio / trios
video / videos
zoo / zoos

Some regular countable nouns ending in
o form their plurals by adding either -s or
-es, though -es is usually preferred:

grotto / grottoes (grottos)
memento / mementoes (mementos)
mosquito / mosquitoes (mosquitos)
motto / mottoes (motts)
tornado / tornadoes (tornados)
volcano / volcanoes (volcanos)
zero / zeroes (zeros)