Never buy a OnePlus phone ever again. They now have a hardware anti-rollback fuse that blows if you revert to an earlier version or install a custom ROM.
https://consumerrights.wiki/w/Oneplus_phone_update_introduces_hardware_anti-rollback
@davidgerard people shouldn't buy OnePlus in the first place, because holy shit what garbage devices. Which get completely abandoned in 12 months or less.
My last two phones have been theirs because they are well supported by LineageOS. I'd still be using the 5T if it hadn't taken an unexpected trip through the washer dryer. The 9Pro that replaced it is also decent hardware.
Their Android version is awful, but easy to replace.