Always on Display is a feature of some flagshipped devices like Samsung Galaxy S7, S8 and Galaxy S9, Google Pixel 2, Pixel 2 XL, etc. OnePlus 6 had this feature built-in, but removed with a software update due to battery consumption issue. However, you can get back Always on Display on OnePlus 6 and even on OnePlus 5 and OnePlus 5T, if root access on the phone.
Always on Display (AOD) will show the clock, notifications, missed calls, etc, even if the phone display is off. It means you don’t always have to turn on the whole display just to check the time or see if you have notifications. Since the information is displayed in a plain white text and in a black background, the battery usage will be negligible. This is because these devices are using AMOLED display and black pixels do not consume battery.
Since OnePlus 6, OnePlus 5 and OnePlus 5T uses AMOLED display and similarity in OxygenOS, interested users can enable Always on Display without affecting battery life much on these devices, but it requires root access and Magisk installed on the phone.
How to Enable Always on Display on OnePlus 6, OnePlus 5 & 5T
1. First, root your phone using latest version of Magisk (Guide)
2. Download the Magisk Module from this page.
3. Using the Magisk Manager app, install the downloaded module
4. After installing the module, reboot your phone
After rebooting your OnePlus 6, OnePlus 5 or OnePlus 5T, you can see the always-on display feature under display settings menu.
Source: (XDA, https://bit.ly/2H0FaKs , techtrickz: https://bit.ly/2Jh2NU0)