A couple of days back, OnePlus released stable Android 8.1 Oreo based OxygenOS 5.1.2 update for both OnePlus 5 and 5T with May 2018 security patch, double-tap to lock screen gesture and several improvements. The OxygenOS beta build allows users to try out new features before they arrive for everyone.
Changelog: The update is now rolling out through OTA for beta participants. If you would like to install OxygenOS Open beta manually and want to enjoy the latest features of Android 8.1 Oreo based OxygenOS Open Beta 11 or Beta 9 on your OnePlus 5 or OnePlus 5T, then you can download and install the full ROM. You can download latest OxygenOS Open Beta build from OnePlus download section or from this OnePlus forum thread. After downloading the update file, install the update via stock recovery as described here. Keep in mind that since this is a beta OS, it is possible you will run into bugs and other instabilities. (Source – OnePlus)
