One Plus One CM12 update always reboots and starts the "Android is Upgrading..." process
Did you perform a full factory reset or did you do a "dirty flash"?

If it is the latter, make sure that you perform a full factory reset, including the cache, and then re-flash; that should solve your problem, it did for me at least.

If that doesn't work I reccommend using the latest build of BlissPop which is built on CM12.1 and runs like butter.