1996 Cadillac Fleetwood Brougham O2

I have had many of the 1996 Platforms over the years, Caprice, Impala SS, Roadmaster, and Fleetwoods.
I like 96 because of the OBD II System.
BUT Recently had a misfire from my 96 Fleetwood, and it said the downstream 02 On the pass side was bad.
So I replaced it, and it ran even worse, so I took out the brand new one, took my old one (which the vehicle was running well with, just a couple misfires) and wire-wheeled it VERY GENTLY, and put new Antiseize on the threads, reinstalled it, and now the vehicle will not run.
It coughs as though it wants to start, but sadly it's a no-go.
I have swapped the new and old 02 sensors back and forth twice, to see if somehow in someway I was missing something, but to no avail.
She won't actually run.
I had to step away to regain my sanity.
I have had I would say 9 of these vehicles with the identical systems, and this is the first time I have ever come across something like this.
Your feedback is MUCH appreciated.