So all these codes at once, all of a sudden?
 

I realize they are 02 sensor related codes, but they've popped up all at the same exact time and for seemingly no reason, as the Disco hasn't been driven in a good month or so until today. What gives? Where should I start looking first?

P0134
P0135
P0155
P0161
P0414
P0000

check your SAI first if you are having some leak that triggers the O2 sensor fault.

Don't have SAI on my truck unfortunately.

Weird! The P0414 code is a SAI code.
Two of the other codes are are for o2 heater circuits. If the heaters aren't working, they will often give low or no activity codes. Try checking the fuse for the o2 heater circuit. If the fuse is good, check for power at the o2 sensor connectors.