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.
um a i orum o is um a o is
A common mnemonic for remembering the chemical formula for water (H₂O) is "Two H's and an O," which highlights that water consists of two hydrogen atoms and one oxygen atom. Another mnemonic is "Water is H2O, just like a pair of shoes (2 H's) and an O (one sole)." These phrases help simplify and reinforce the structure of water in a memorable way.
es, like echo- e choes that would be plural
Most of the time just -s.
o
-s. Examples include radios, videos, and pianos.
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 two plurals of "vortex" are "vortexes" and "vortices".
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.