Recently, OnePlus released stable Android 8.1 Oreo based OxygenOS 5.1.0 update for OnePlus 5 and 5T, while the beta build allows users to try out new features before they arrive for everyone. For example, the OxygenOS Open Beta 9 and 7 bring double-tap to lock screen support to both devices.
Changelog: 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 9 and 7 on your OnePlus 5 or OnePlus 5T, then you can download and install the full ROM. You can install the update via stock recovery as described here. However, keep in mind that since this is a beta OS, it is possible you will run into bugs and other instabilities. (Source – OnePlus)