If you are planing on buying nawet phone you should know its IMEI number, becouse you can find exact information about your phone with it, and also, you can check if your device is blacklisted. You should also memorize you number, to blacklist it, in case it gets stolen.
How to find IMEI number of OnePlus device? Below you can find several methods.
Using the Dial Pad Enter *#06# in the dialing field. All IMEI numbers should instantly pop up on your screen.
In the phone menu Go to Settings and then About phone -> Status
On the box IMEI number is labeled on OnePlus box
On the SIM tray You can find IMEI number labeled on the SIM tray
Below you can find a video showing how to check IMEI number on the example of OnePlus 7 Pro.