OnePlus 5T was launched back in 16th Novemberand it took no time for OnePlus to send in the first ever beta update of Android Oreo. I still remember the time when everyone was critising OnePlus for not including Android Oreo in OnePlus 5T, but turning out all the odds just in a 45 days, first beta update has been rolled out.
Since, this is only a beta update so it is not available to all the users. OnePlus would choose some users for this beta update, so if you want to get this beta update of Android Oreo, then I suggest you sign up to OnePlus account
Changes in OnePlus 5T after Android Oreo beta update
Optimized the app shortcut style
Combine icon options with app shortcuts
Now able to upload photos to Shot on OnePlus
Added Picture in Picture
Added Smart Text Selection
New Quick Settings design
Updated Android security patch to December
You will need to flash it manually by downloading the ROM that is 1.6GB size from downloads.oneplus.net and the company has also provided flashing instructions, which you can check out in the source link below. You should get the OTA update automatically from second beta. OnePlus had already promised final Android Oreo update for the phone by early 2018.