Actually, not all native French speakers say 4*20+10+9 for 99. In Belgium and Switzerland, they have a word for 90 that isn't used in France (nonante). Thus they say 90+9 for 99 (nonante-neuf). The same goes with 70 (septante instead of 60+10 soixante-dix).
I'm from France, so you can imagine how I felt the first time I heard those words and realized I had always said 60+10 for 70 and 4*20+10 for 90 ;-)
In France too, in some areas, it's used. It's no more the official way taught in schools to say it, that's all.
The "quatre-vingt" is the Parisian way, and it becomes the only national way, I guess with the national TV broadcast and newspapers. And the "octante", "nonante" was the old way to tell it, in non Parisian areas. My grand-mother always said "nonante", and she was 100% French.
It's the opposite for me. My grandparents always taught me "nonante", "septante", and at school I had to learn a new way. But when you are a little girl, you don't care about "nonante" being more logical than "quatre-vingt-dix", you only learn it for 90, and that's all. You become to think about the language and its etymology far later in your life.
It's not "or", it's "aur". So no more headache when you write it down on your notebook.
"o" and "au" are different o, one is the close o, and the other the open o.
Surprisingly it doesn't really create any confusion in day to day Hinglish speech.
The only place where it became a headache was the computer class in High school with the and/or/not bridges of computer logic. The teachers generally speak Hinglish and there was total confusion about what he meant when he said Or.
We resolved it by a mutual understanding that all technical terms would be assumed to be in English only.
You have to forget the "o" in the English "or".
You have to think the IPA way, to write the sounds as they are, and not the letters.
You are already level 11 in French, and French will help you a lot with this matter.
In IPA, there are 2 kind of "o". The closed o, and the open o.
The closed o
High pitched "o", a real, pure o sound.
Closest in English: no, soap.
In French: tôt, atome, gros, mot...
The open "o" noted here "au" (and in French too)
In English: law, caught, all, halt, talk
Official English IPA: lɔː, kɔːt, ɔːl, hɔːlt, tɔːk
I know it depends on your English dialect, but it's a lower pitch "o". In French: in "bol", "bonne", "Paul".
Listen to them: