Xiaomi has released first Android Oreo based MIUI Global Beta for Mi 6. The Global Beta ROM is MIUI 9 v 8.1.11 that is based on Android Oreo.
In order to install the Global beta on the Mi 6 device, the users have to upgrade their Mi 6 devices to MIUI 9 Global Beta ROM v 7.12.14 which is based on Android Nougat using the recovery flashable zip file. After installing the version, users will get a notification for an OTA update for the Android Oreo Based Global Beta. The ROM is currently in beta stage, and soon you will get the stable version of the same.
If you want to install manually, you can download the ROM here.