‘O’ has an interesting backstory. The shape comes from phonecian ayin and has the meaning of “eye”, from which it takes its form. It probably had a sound close to the Arabic ayin (a kind of guttural growl where you half choke yourself).
However the Greeks were probably just as confused by that sound as non-Arabs are by it today, so they instead gave it a vowel sound.
‘A’ had a similar story. In Phonecian it represents a glottal stop and has the name ‘alep. However a glottal stop at the start of a word is quite hard to hear if you aren’t used to it, so the Greeks seemingly assumed that the letter represented the second phoneme of ‘alep, an /a/ sound. Hence the alphabet was invented, a mixture of vowels and consonants with equal status.
It's really a lucky happenstance that Semitic languages have enough gutterals that could be repurposed as vowels; if these had been lacking, or if Greek had needed them as consonants, then it's easy to imagine a sort of incomplete consonant-only orthography coming into use, perhaps even persisting to Latin alphabets in the modern day. Greece's first introduction to writing (linear b) didn't stick and doesn't seem to have been used for anything other than palace records, arguably because of deficiencies in that script, might literacy in the West have similarly been stunted by scripts that could not show vowels?
Losing the vowels works in languages like Arabic because of the structure of the language, with consonantal clusters (e.g. ktb, tlb) being semantic “roots”. So you usually know from context, and if you don’t know the absolutely precise word, you know the meaning, which can sometimes be more informative.
And Greek too has two versions of the lower-case sigma: σ (whose usage corresponds to that of the long s) and ς for use only in the word-final position (corresponding to the terminal s as distinguished from the long s).
In general I’m glad we use Leibniz’s notation and not Newton’s, because my handwriting is messy. Though I did use dot and dot dot in physics long ago.