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.
Changelog: 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)
