After some of the most recent devices from Xiaomi were updated, even the older Xiaomi Mi5 and Mi Note 2 can now run the latest Android Oreo version. The update to Android Oreo has just been released under the Global Beta ROM. It is not the final stable update, so it does not arrive via OTA on the Global Stable firmware. Even if you own one of these two phones and you have the Global Beta ROM installed, you will still need to flash Android Oreo manually.
On both the Xiaomi Mi5 and Xiaomi Mi Note 2, the update to Android Oreo is based on the MIUI 9 which comes in the version 8.2.1. The firmware includes the security patch from the month of February, which is very important because it fixes the Spectre and Meltdown vulnerabilities.
The update also includes several bug fixes for issues present in the previous version of the firmware. It is possible to flash the firmware manually via fastboot and TWRP, but you need an unlocked bootloader in order to do it. If you do not want to install the Global Beta on your Xiaomi Mi5 or Mi Note 2, you can wait for the final version which should come out in the next couple of weeks.