After being rolled out OxygenOS 5.1.0 stable update for OnePlus 5 and 5T, the OnePlus team just released a new update for OnePlus 3 and 3T through the open beta channel. OxygenOS Open Beta 35 and Open Beta 26 for OnePlus 3 and OnePlus 3T come with double-tap to lock screen feature, enhanced App drawer search history, and more.
OxygenOS Open Beta 35/26 for OnePlus 3/3T is still based on Android 8.0 Oreo. There is no information about Android 8.1 Oreo update for both devices, while OnePlus 5 and 5T have already upgraded to Android 8.1 Oreo (OxygenOS 5.1.0). Below is what is new in this update.
Supports double-tap to lock screen
Improved search history for App drawer
Hide lock-box entry in guest mode for File manger
Optimized scanning logic for large files
OnePlus 3/3T runs OxygenOS Open Beta channel will receive the new Open Beta 35/26 update through OTA. Other users can download the factory image and flash onto the device manually via Fastboot, which requires ADB and fastboot driver setup on the computer.
If you would like to flash the OxygenOS open beta on your OnePlus 3 or 3T, then download the latest build (Open Beta 35 or Open Beta 26) from OnePlus download page and follow this flashing instruction. As always, back up all important data you have on the phone before proceeding to flash the new firmware.
Keep in mind that since this is the beta OS, it is possible you will run into bugs and other instabilities, so inexperienced users should not try it. (Source)