For nouns ending in O
1. For some nouns ending in O, add -es to the end of the word.
hero - heroes
mosquito - mosquitoes
tomato - tomatoes
zero - zeroes
2. For nouns ending in O that are short forms of words, add -s.
bio (from biography) - bios
dynamo (from dynamoelectric machine) - dynamos
hippo (from hippopotamus) - hippos
photo (from photograph) - photos
piano (from pianoforte) - pianos
zoo (from zoological garden) - zoos
3. For other nouns ending in O, you can add -s or -es.
avocado - avocados or avocadoes
domino - dominos or dominoes
echo - echos or echoes
flamingo - flamingos or flamingoes
grotto - grottos or grottoes
mango - mangos or mangoes
tornado - tornados or tornadoes
volcano - volcanos or volcanoes