@emphrath I also encountered this issue yesterday. I picked up a OPO second hand which had some version of CM on it.
I followed all the steps everyone else did, flashing the firmware files from the latest build and trying several release channels.
I had SIM card but no wifi at one point, then switching channel and I lost the SIM.
What I eventually did was restored the original OxygenOS firmware on the device from http://oxygenos.oneplus.net/OnePlus_Bacon_OxygenOS_201601190107.zip and had everything working again in Android. I then started anew. I chose the development channel and voila everything is working now.
I'm unsure what happens behind the scenes that's made a difference in my case I'm just happy to be working with it already!