Oreo appears for the first time in Google's Android distribution chart9
Although many Pixel and Nexus devices have already been updated to Android Oreo, they account for less than 1% of the Android smartphone market share, which is why they don't matter that much in the grand scheme of things.
Add to that that Google's doesn't take into consideration any Android versions with less than 0.1% distribution and we might have the answer to the question of why Oreo hasn't made it to the charts earlier than October.
In any case, the October distribution figures show Android 8.0 Oreo now accounts for only 0.2% of the market share, while Android Nougat has exactly 17.8%. Marshmallow continues to be at the top of the list with 32% market share, followed by Lollipop with 27.7%.
The hierarchy ends with Android KitKat (14.5%), Android Jelly Bean (7.6%), Android Ice Cream Sandwich (0.6%) and Android Gingerbread (0.6%). According to Google, the numbers are based on the data collected during a 7-day period ending on October 2.