OnePlus has today released Open Beta 1 for the OnePlus 5T. the latest build is based on Android 8.0 Oreo update and brings features including Picture in Picture mode, New Quick Settings UI, Android Security Patch to December and More.
Do take a note that the development of Android 8.0 Oreo for OnePlus 5T is still in early (Beta) stages. hence, these builds are not stable as official builds which are pushed over-the-air.
Source – OnePlus