There are consistent problems with using Bluetooth on the OnePlus One.
In both the official /e/ builds and LOS 15, the phone crashes and reboots when attempting phone calls with a bluetooth device connected.
So I had been using an unofficial pie based build. The phone is stable with the pie version, but using a bluetooth device for phone calls is still not an option because the sound quality is horrible for the person at the other end of the call. (The sound is good for me in the earbuds on phone calls and podcasts.)
I am now using an AOSP 10 rom because bluetooth phone calls work beautifully with it.
It’s clearly problem with LOS and /e/ because I get consistent results in both with 2 different OPO phones and using two different brands of bluetooth headsets.
Before reporting a bug, has anyone else tried using a BT device with phone calls on a OnePlus One?