O is a circle, such a basic symbol that it also represents the mathematical nothing. If you look at the alphabet gif, our O is the same in Etruscan, the language of the people who passed on the symbols (although not the language) to the Roman Empire, which would one day spread it to England and make its way through the years to us. That symbol came from the Greek omicron, where it’s the fantastical symbol…O. And lowercase o. Shocking, right? But interesting side note: omega (Ωω) is also an O in Greek (mega and micron, big and small). It used to specify the long o vowel while omicron was the short, but these days they’re mostly the same.
Back to business. The Greeks came up with their alphabet by copying that of the Phoenicians, who used the symbol O, but not as a vowel. See, the Phoenician language is what’s known as an abjad, or consonant alphabet, meaning they had no symbols for vowels—making it the “oh” sound was the Greeks idea. The Phoenician O, or Ayin, did not symbolize a sound at all. Way backwith the letter A I mentioned that the Greeks made a letter from the symbol for a glottal stop (basically it’s like not saying a hard consonant, like t, before another consonant (“pet dog” becomes “peh dog”)). Anyway, the Greeks did the same thing with O, this time taking a symbol for a voiced pharyngeal fricative. I can’t really explain what that is, but they have an audio example on the Wikipedia page for it. It’s something like “aaah”.
The voiced pharyngeal fricative (say that three times fast) is also how the Proto-Sinaitics the Phoenicians descended from used the letter. It also means eye, although the letter is pronounced something like ‘en or enu. Sure enough its original symbol was a flattened oval or an elongated one with a dot in the center. They took the symbol from Egyptian hieroglyphs, and the Proto-Sinaitic word for eye was attached to the Egyptian symbol for it.
TL;DR: More than four thousand years ago O was an oval with a dot, but since then it’s been a circle, even when it didn’t represent the sound we know it as.
“Letter Perfect: The A-to-Z History of Our Alphabet” by David Sacks.
And Wikipedia. But just for the sound! It wasn’t research, I swear.