After upgrading to Android Oreo OxygenOS beta 1, you may try to flash any mods, root or custom ROM but it may show you an error in the TWRP Recovery which shows Unable To Mount Storage or Failed to Mount. This means that the latest TWRP Recovery doesn't support the Android Oreo beta 1 update on your OnePlus 5T. In this guide, I will help you to Resolve Unable to Mount Storage Problem On OnePlus 5T after 8.1 Oreo (Open Beta 1 or later). Even though if you are running the Official TWRP Recovery, you may experience this problem.
How to Resolve Unable to Mount Storage Problem On OnePlus 5T after 8.1 Oreo
You will have to flash the new TWRP Recovery that we shared here. Thanks to the XDA Senior Member eng.stk, for sharing the blu_spark TWRP Recovery for both OnePlus 5 and 5T.
Install the blu_spark TWRP Recovery on your phone to fix this problem on your phone. (Download from Here)