This is just wrong. The correct wording is they made a custom ROM from the stock Android provided for free by Google(AOSP). They didn't create an OS. Every phone manufacturer apart from very few phones use custom ROMS(except akina Pixel,Nexus,Android One phones).
However, the Oxygen OS is among the best Custom ROMS out there as it is close to stock Android plus additional features that don't slow it down like the Touchwiz that people keep saying each year that it is lighter and faster yet after a year, it is slow af.
If u have the cash the best phone is called One Plus 3T has a 64gb and 6gb ram..its extremely rugged I have used mine for 1yr now..I hear its now come down to 40k
There is also the newly released One plus 5T Sold at 65k with 8GB and 125Gb HD all one plus Android are Android 8 Oreo and full HD display.
Believe me it's worth every shilling. No freezing or hanging no overheating, battery takes me for a full 24hr after fast charge of 0%-100% in 30mins. Comes with a special charger called dash charging and the company rolls out updates regularly.
They even created their own mobile operating system called the Oxygen OS.
It's a good mobile if you want to spend reasonably.
Contact me for additional info if u need to buy from a genuine dealer in Nairobi.