If the battery is disconnected, you have to drive ATLEAST 100 miles before all the sensors work properly. The mileage varies for different cars but you have to drive a certain distance. I have cars fail all the time because of MIL readiness.
They would of had to disconnect the battery to install an alarm, so just drive it around alot and you'll be set!
P.S. MIL Readiness has NOTHING to do with an oxygen sensor lol!