My 3T just said there was a new OS update (Oxygen, or whatever OnePlus call it). It downloaded fine and appeared to install fine.

 

But after reboot it said "Enter password to start Android".

 

Other than my fingerprint or the PIN I've never used a password and the PIN, nor anything else I can think of, will let me in.

 

I tried the obvious such as "password", 0000 and 1234 and it eventually said "to start the phone, turn it off and back on again".

 

I rebooted and it seems to be ok.

 

Anyone had this before ? It just appears to be a November 2018 security update.