OxygenOS 9.0 is rolling out on OnePlus 6 devices, and the OS will be available later on OnePlus 3, 3T, 5 and 5T. With Project Treble, Google is hoping to reduce OS fragmentation across the Android ecosystem (as of earlier this month, around three quarters of devices were running Android 7 or earlier). So, perhaps that effort has helped OnePlus to roll out its fork of Android Pie so quickly.