There's not a good one. You just have to memorize the "oddball" words that take "es" as an ending. Most words ending in "o" will just take an "s" as the ending. Use a Study Deck - see the link - to memorize the other words. Here's another good link to show you some of the most common plural endings for "o" words, too.
O.Henry was a master of Surprise Endings to his stories.
-s
um a i orum o is um a o is
Plurals for words ending in o can end in -os or -oes. For example, "potato" becomes "potatoes" and "photo" becomes "photos". It depends on the specific word and its language of origin.
o
Most of the time just -s.
pluralsThe word plurals is the plural of plural. As in: I answered the question about plurals to the person who didn't know that the plural of plural is plurals.
The plurals are boxes, sheep, and princes.
I think its a feminine noun. Many languages have gendered nouns, like in spanish, O endings are masculine, A endings are feminine.
O. Henry, the pen name for William Sydney Porter, is known for his ironic and unexpected endings in his short stories. In "After Twenty Years," the twist at the end reveals the true identity of the characters, while in "A Service of Love," the conclusion presents a surprising revelation about the characters' intentions.
Hermoso/a, lindo/a, bonito/a, bello/a. The "a" endings are for feminine subjects, the "o" is for masculine.
pluralsThe word plurals is the plural of plural. As in: I answered the question about plurals to the person who didn't know that the plural of plural is plurals.