Available courtesy of Android Police, the APK weighs 10.74MB and should work on the OnePlus 5, 5T, 6, and 6T. Some noted that they even got Zen Mode working on the OnePlus 3T running the beta build of Android 9 Pie.
If you own an older OnePlus phone running Pie, go here on your phone to download the Zen Mode APK. Once you download the APK, make sure Chrome allows you to install unknown apps. If you didn’t allow that, you’ll see a prompt that lets you change that.
Open Zen Mode and make sure to read the instructions and detailed explanation of Zen Mode. The mode is serious when it says you can’t use your phone for 20 whole minutes, so make sure you’re ready before you tap the Let’s Go button at the bottom.
While in Zen Mode, you can only answer calls, make emergency calls, and open the camera. According to Android Police, the camera button didn’t work on the OnePlus 6T. You might or might not encounter a similar issue on your device.
Also while in Zen Mode, the phone mutes incoming notifications and prevents you from using it for anything other than the aforementioned functions. Once the 20 minutes are done, Zen Mode shows you how many times you’ve used it, how many notifications it muted, and the total amount of time you’ve placed the phone in Zen Mode.
Unfortunately, those without a OnePlus device are out of luck. I tried to grab the APK for my Google Pixel 3 XL, only for the phone to refuse to install it.