After upgrading to the third-party Oreo OS, Xiaomi MI5 official releases news in MIUI International forum to announce its first MIUI 9 International version OS based on Android8.0 O OS, it is MIUI 9 V8.3.1, the users who like Android 8.0 OS can download to use first.
It is worthwhile to note that before upgrading, please save personal data as backup first, they suggest USB cable to update not OTA, or parts of functions can not work normally.
Update Changelog:
Android Oreo
Fix the issue of the low voice when make phone call
Fix the issue that the notification bar content is covered black.
Fix the issue that save the contacts to show the digital number in phone book record.
Fix the issue to use Xiaomi transferring to cause re-switch on issue at random.
Fix the issue to use middle fonts to cuase the bookmark and download management lost
Fix the issue that can not show color notification normally.
Recovery Upgrade: Download here
USB cable upgrade: Download here
Source: Xiaomibuying