There is a bug report for this issue here:
OnePlus One wifi switch after OTA6 #995
Specifically this comment:
One needs to enable developer mode in settings. Then connect with a device with adb tools installed.
Once you're in the shell:
sudo mount -o remount,rw /
That is to mount the root partition read/write. Now you can replace or edit the WCNSS_qcom_cfg.ini file. The one mentioned in #995 was sufficient to get wifi working for me.
Hope this helps