OxygenOS Open Beta 8 and Open Beta 6 are based on Android 8.1 Oreo and an important change in this update is April 2018 security patch, which ensures better security. Alongside general bug fixes and improvements, the update also includes some UI changes to clock, fixes QR code scan issue of OnePlus Switch, auto play in headphone mode, and more.
Changelog: If you would like to try the new features of OxygenOS on your OnePlus 5 or 5T, then you can install the latest OxygenOS Open Beta 8 for OP5 or OxygenOS Open Beta 6 for OP5T. Keep in mind that this is beta OS, so it is possible you will run into bugs and other instabilities. Auto Play– Pause music when connected to headphone Added Notification ringtone options Added Broadcast caller information when using headphone OnePlus Clock: Changes to Alarm background color and other UI changes Stopwatch updated with new UI OnePlus Switch: Fixed QR Code scan issue General bug fixes and stability improvements Card Package: Added a new membership card in Shelf System: Updated to Android security patch to 2018-4 General bug fixes and improvements If you are interested, download OxygenOS Open Beta 8 for OnePlus 5 from here or OxygenOS Open Beta 6 for OnePlus 5T from here. Once you have downloaded the factory image, follow this installation guide. (Source)