Depending on your region, you can not be offered the same call recording options that other OnePlus users have. But as with most things Android, where there is a developer with a will, there is a way no matter. And the method we will describe is one of the easiest, most stable and undiscovered ways to do that. Perhaps we should discuss why OnePlus has kept you out of this feature.
Have you ever heard this message when calling customer service: "Your call can be recorded for quality and training?" It is likely that they will notify you that you are recorded because it is the law at that place (or at least it is necessary for the recording to be admissible in court). All OnePlus devices are sent from the factory with the ability to record calls, but based on information about your region, this option may be disabled or appear completely unavailable. You can change it with this mode, but you still want to be responsible for following the laws of your region.
Before you start recording calls, first check if your state is legal. While federal law allows registration of phone calls and personal conversations with the consent of at least one of the parties (eg you), these laws may vary from state to state. For example, California has a two-party consent, which means that you must get permission from the other person before you can start recording. 
To start, open Magic Manager and then open the navigation menu by either swap left or right or press on the 3 stacked parallel lines in the upper left corner. Touch "Downloads." Start typing the name of the module – the word "call". When you see the "OOS Native Call Recording Enabler" option below, press the download icon (down arrow pointing to a horizontal line.)
Next, press "Install" and Wait for the file to flash. You have already downloaded the file, so you do not have to do it for the second time. You will see the flashing status of your screen, with the ability to reboot when you are done.
This Magic Module can survive a reboot, and actually requires activation of the recording function. Even if you accidentally shut down the reboot mode on the flashing screen, press the power button for a long time until you see the option to reboot. Confirm that you want to reboot. Now that you open modules, you should see the OOS Native Call Recording Enabler listed.
To enable your new recording function within your phone (dialer) -app: Start by pressing the menu button in the top right corner. Select Settings. Select "Call Record" and select the option to "ON" mode. After pressing the switch, you will see an additional option to enable automatic playback of calls.
If you selected manual call recording, you can record conversations with a new tape recorder in the call. Records are saved as MP3 files in a folder on your internal storage partition named "record."