What's that saying, "with great power comes great responsibility"...?
If, like me, you've rooted your OnePlus One, you may experience issues with the latest over-the-air software update from Cyanogen. To be honest the last two updates, for whatever reason, haven't gone well for me. So as a public service I'm posting the steps I took to return my OPO to a state where it would accept an OTA.
If, like me, you tried flashing the update manually and that didn't work either, then I'm afraid I've some bad news: you're likely going to have to (1) factory reset your phone, and if that doesn't work (2) re-flash the factory image from Cyanogen. The good news? By the end of this lengthy process you'll have backed up pretty much everything on your device, which you should do from time to time anyway.
This guide will assume that you've figured out how to get your computer and OPO to talk to each other via adb and fastboot. The process for doing so is different for Windows, Mac and Linux computers but I believe the command prompts that you'll see below are the same. I use Linux because Linux is awesome.
Ready? Let's do this!