https://wccftech.com/oneplus-6-and-6t-get-miui-10-based-on-android-pie-port/

Thanks to the open source nature of Android, it isn’t uncommon to see AOSP-based builds for a variety of devices. However, it’s quite rare to see OEM ROMs ported to other devices. We don’t see a Samsung device running EMUI or a Pixel running ColourOS. Cross-posting ROMs between OEMs is a feat in itself, and today, Xiaomi’s OEM ROM MIUI has been successfully ported to OnePlus 6 and OnePlus 6T by an XDA Developers member. This MIUI 10 port is based on Xiaomi’s Android Pie build and it requires your OnePlus 6 or OnePlus 6T to have the OB8 or newer firmware installed before you can install it. Since this is a port, some features might work and others might not. Here is the list of bugs that the developer is aware of:

MIUI’s Always On Display works, themes, static/live wallpapers, and ringtones work. Also, full-screen gestures and Mi AI work as well. So if you’re a fan of MIUI 10 or just want to try it out on your OnePlus 6 or OnePlus 6T, you can sideload it on to your device. Bear in mind you’ll need a third-party recovery such as TWRP to get the job done. Download the ROM using this link, and NFC/Audio fix using this link. More details can be found in this thread. The port is not daily driver material by any means, as a lot of apps might not work as intended.