Hi!
Did you try to reflash the phone using twrp and the latest official .zip? Theoretically speaking it should be possible to copy the current upgrade https://www.oneplus.com/de/support/softwareupgrade/details?code=PM1574156097213 onto a sd card or the internal storage of the phone. Flash the phone with twrp recovery ( https://twrp.me/oneplus/oneplusone.html) , reboot to recovery and flash the zip from within the twrp recovery.